compressed air piping selection and design - esl-ie-05!05!10

Hank van Ormer AIRPOWER USA 11520 Woodbridge Lane President Baltimore, Ohio 43105

MODERN COMPRESSED AIR PIPING SELECTION AND DESIGN CAN HAVE A GREAT IMPACT ON

YOUR COMPRESSED AIR ENERGY DOLLARS

This paper introduces new concepts in compressed air piping, sizing, and system design beyond the conventional pipe sizing charts and standard system layout guide lines. The author shows how compressed air velocity has a very significant impact on the pressure losses and piping performance. Case studies are used to show how conventional piping design and sizing keep “extra compressors on line” – preclude proper control operation – waste energy – shorten filter life – and have a negative impact on dryer performance. The principles offered in this paper will help to trouble shoot old systems and help design new systems. They will also help to avoid many common mistakes made over the last 30 to 35 years with the ascendance in popularity of the rotary screw compressor package.

Prepared for

Prepared by

Hank van Ormer

President

AirPower USA, Inc. 11520 Woodbridge Lane

Baltimore, OH 43105 (740) 862-4112

www.airpowerusainc.com

ESL-IE-05-05-10

Proceedings of the Twenty-Seventh Industrial Energy Technology Conference, New Orleans, LA, May 10-13, 2005

Have Any of These Things Happened to You With Regard to Your Compressed Air System?

Plant personnel say the current 200-hp compressor is too small and won’t hold the system pressure. You purchase another 200-hp unit– things get better but still seem to be marginal? What’s wrong? You doubled the air supply! Did you double production? If not, you should. The cost of your “most expensive utility” – compressed air – has significantly increased! The compressed air supply is on the north end of the plant. You have trouble holding pressure on the south end. Those “in the know” claim that because it’s so far down there, we need to either “raise the compressor discharge pressure” (a significant energy cost increase) or “install another air compressor on the south end” (often the first step in upsetting the system timing and efficiency! The pressure problem isn’t the distance – it’s either the pipe sizing or system design or both. The most economical fix is usually a well-planned piping modification, which is a “one time” cost. The other solutions may have a high initial cost or not – but the related electrical energy cost increase goes on year after year. You run four lubricant-cooled rotary screws (total 400 hp) during full production for two shifts and during the third shift, you still run the same four compressors (400 hp), even though the production is 50% or less during the third shift. You may run the same on weekends and holidays – no one is sure. You have tried shutting a unit off, but sometime during the shift, it has to be manually turned back on in a hurry – so you leave it on. The units have automatic start/stop controls, but they never turn off automatically. You purchase a new central control energy management system with great promise. You are still running most of the units most of the time. This is a more complex issue with several factors, but the root cause of the issue is often the pipe size and layout changes. This list could go on and on with issues and opportunities fundamentally set up with improper or poorly designed compressed air piping systems. Over the last almost 20 years, we have reviewed almost 2,000 compressed air systems for energy reduction and production and quality improvements. Our theories and ideas are not “theoretical” – they are based on field observation, and most importantly, on pre- and post-project measurement. The ideas and suggestions we present are not new – many have been lost or forgotten in transition. Compressed air is not complex, but the physics of compressed gases and the knowledge of the operating parameters of the equipment must be utilized with common sense. Pipe size and layout design are the most important variables in moving air from the compressor to the point of use. Poor systems not only consume significant energy dollars, but also degrade productivity and quality. How does one properly size compressed air piping for the job at hand? You could ask the pipe fitter, but the answer probably will be, “What we always do”, and often that’s way off base.

AirPower USA, Inc. 1 www.airpowerusainc.com

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Another approach is matching the discharge connection of the upstream piece of equipment (filter, dryer, regulator, or compressor). Well, a 150-hp, two-stage, reciprocating, double-acting, water-cooled compressor delivers about 750 cfm at 100 psig through a 6-inch port. But most 150-hp rotary screw compressors, on the other hand, deliver the same volume and pressure through a 2-inch or 3-inch connection. So which one is right? It’s obvious which is cheaper, but port size isn’t a good guide to pipe size. Charts and Graphs Many people use charts that show the so-called standard pressure drop as a function of pipe size and fittings, which sizes the line for the what is referred to as an acceptable pressure drop. This practice, too, can be misleading because the charts can’t accommodate velocity- and flow-induced turbulence.

Some might call pipe sizing a lost art, but we see the issue as a lack of attention to detail, basic piping principles, and guidelines. Read on to learn how to size air piping using velocity, which when combined with appropriate piping practice, ensures an efficient compressed air distribution system. As compressed air system consultants and troubleshooters, we use certain guidelines to design new piping systems and to analyze existing system performance and opportunities for improvement. There are four different categories of compressed air piping. They all have a job to do, and therefore, require different design criteria:

• Inlet piping • Interconnecting piping of the air supply • Distribution piping – headers/sub-headers • Piping to feed the process.

As we look at each of these groups, we will list some of the “rules of thumb” or guidelines, which are appropriate. Inlet Piping Guidelines: It is important to air compressor efficiency and integrity that the inlet is as cool as possible, at as high pressure as possible, and free from contaminants such as dirt, pollen, birds, etc. and water.


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Inlet piping is often ignored today, because many of the fully packaged units have the intake inside or on the package and take in room or ambient air. However, some plants will remove the room air intake (particularly when it is contaminated) and install a pipe to an outside-mounted filter for a more appropriate air source. Many larger compressors, such as centrifugals or oil-free rotary screws, will have cooler outside inlet where the cooler inlet air has a much higher impact on performance efficiency then a lubricant-cooled rotary compressor. When this happens, what size pipe? Don’t size the pipe by the size of the inlet opening! The objective is to deliver the air to the compressor inlet at the highest possible pressure after it leaves the outside intake filter or pick-up point. Like all piping, the pressure loss is a function of flow (scfm), pipe material, distance, and turbulence. Size for negligible pressure loss by function – it doesn’t usually cost much to step up to the next size.

It can be good practice to provide a large volume in the suction lines as shown here, just before the compressor intake flange. This will:

• Act as a pulsation dampener

• Trap any condensation (be sure to install drain and trap).

Material: The proper inlet pipe brings the air from the filter to the compressor with no pressure loss and should not create operational problems with any type of self-contamination on the inside. It is important to realize that the ambient inlet air condition may well dictate the selection of one type of pipe over another. GALVANIZED INLET PIPING has the advantage of resisting corrosion better than standard iron pipe. However, over time when the corrosion does set in, the galvanizing material then peels off. The inlet pipe is now a producer of potentially very damaging, solid contaminants between the filter and the compressor. This would be particularly dangerous to the mechanical integrity of a centrifugal compressor. During high humidity weather, it is quite conceivable that condensation will form in the inlet pipe (therefore, the OEM installation manual should recommend a drain valve be installed on the pipe before the inlet). Condensation in the pipe will obviously accelerate the time frame before the coating breaks down. This time frame is dependent upon where the thinnest portion of the coating is applied. Stainless steel inlet pipe is the best possible material or such large-diameter, low-pressure inlet air, as long as it is installed properly and the inside is properly cleaned.


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There are also many grades of plastic material suitable for inlet air piping. Summary: We recommend either stainless steel or proper plastic-type material for inlet piping and do not recommend galvanized piping. Design: Here’s where common sense comes in: Tight turns may cause pressure loss through turbulence – run as straight and smooth as possible and as direct. We want the air to stay cool – if you have to run the pipe into a hot room, or by a heat source such as a boiler -- then insulate it. With reciprocating compressors, be sure you are not in “critical length.” Support the weight of the inlet pipe – don’t let it hang on the compressors. Assuming the filter does its job on dirt, frogs, etc. as contaminants is there any chance of internal piping condensation. If so, just to be safe, install a drain point at the low point in the inlet pipe just before it enters the compressor. On centrifugals and oil-free screws, we always recommend this drain. Seem simple? All these moves seem logical and easy to follow, particularly having read the guidelines. We have corrected problems in all these areas time and time again in the field. For example, recently in a foundry, we found:

SULLAIR

20-150L

SULLAIR

20-150L

SULLAIR

20-150L

1040 gal

Zurn RA 1400 Refer Dryer

Filter Filter Filter

Separator

Separator

Separator

FOUNDRY – LONG SMALL INLET PIPE

2” 2”

2”

3”

150 HP 150 HP 150 HP

Inlet air to compressors – 6” pipe 260-300 ft measured vacuum 18” Hg = 9.5 psia inlet pressure assuming a normal 14.2 psia/inlet pressure, this lowers the scfm from 725 scfm at 60°F to 501 scfm at 60° -- a loss of 31% volume per unit.

6” - 300 Ft of inlet pipe

To supply required 1400 scfm, the plant had to run one “extra” 150-hp full load – estimated electrical energy cost wasted -- $54,000 /year for 26 years = $1,400,000 wasted energy! Interconnecting Piping This is the piping area where we find the most “opportunities” in compressed air systems, particularly in those installed after the late 1970s. The older systems were put in more carefully and the introduction of lubricant-cooled rotaries created many misconceptions recommended by well-meaning but untrained personnel. Guidelines: the higher the pressure the compressor has to produce, the more electrical energy is required to run (1/2% per psig). With a few exceptions, the most energy efficient point for all


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compressors to run is at full flow. A well-designed and controlled system will have no more than one unit at less efficient part load – all others at full load or off. The piping from the compressors to the filters, dryers, and the air receiver is what we call the interconnecting piping. At today’s recognized energy costs, its job is to get the air from the compressor discharge to the dry air receiver (or system header, if there is no receiver with the lowest possible pressure loss. Part of this loss is filter and dryer selection, which is either implemented well or not. However, we often find significant problems, particularly in multiple compressor installations in pipe sizing and even more in layout or design. Too much backpressure in the interconnecting piping can have many consequences, not immediately obvious:

• Reduce effective storage and cause short cycling in step-controlled units – not only wasting electrical energy, but also shortening the life of coolers, motors, air end, and coupling.

• Keep modulated-type controlled units from being fully loaded and subsequently running

multiple units at plant load.

• Cause the compressor to run higher pressure to deliver appropriate pressure to the system.

Line Pressure -- psig Nominal Pipe Size

Cfm Free Air 10 15 20 30 40 50 75 100 12 150 200 250 300 350 75 .19 .16 .13 .10

100 .28 .24 .20 .16 .13 .11 150 .69 .57 .49 .38 .31 .26 .19 .15 .12 .10 200 1.20 1.00 .85 .66 .54 .46 .33 .26 .21 .18 .14 .11 250 1.53 1.31 1.02 .83 .70 .51 .40 .33 .28 .21 .17 .15 .13 300 1.89 1.47 1.20 1.01 .73 .57 .47 .40 .31 .26 .21 .18 400 2.50 2.04 1.73 1.25 .98 .80 .68 .52 .42 .36 .31 500 3.87 3.16 2.67 1.93 1.51 1.24 1.05 .81 .65 .55 .48 600 4.50 3.81 2.75 2.15 1.77 1.50 1.05 .93 .79 .68 800 4.87 3.82 3.13 2.66 2.04 1.65 1.39 1.20

1000 7.55 5.90 4.85 4.12 3.16 2.56 2.16 1.861250 9.12 7.49 6.35 4.87 3.96 3.32 2.871500 10.8 9.17 7.02 5.70 4.80 4.141750 12.5 9.54 7.74 6.50 5.622000 16.3 12.5 10.1 8.50 7.352250 15.8 12.8 10.8 9.30

2”

Schedule 40

2500 19.4 15.8 13.3 11.4

Most often, inexperienced compressed air system design personnel use only the “Standard Pressure Loss Charts” (see above). These charts reflect pressure loss in piping what we normally call “friction loss” – the air pressure being lost to the friction on the pipe walls. As you see, the chart is in psig loss per 100’ of pipe based on flow (scfm) and entry pressure (psig). This is generally very accurate and satisfactory for distribution air, but in the interconnecting piping, we have to consider another cause of pressure loss – turbulence.


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Pressure Loss in Pounds for Each 100 Feet of Straight Pipe The impact of turbulence on pressure loss is velocity dependent. If the air system designed is not considering turbulence, then they are not up to modern times. Interconnecting piping is usually short runs tying many pieces of equipment together, perhaps with bypasses. It can become very convoluted. Using only the standard pressure drop charts will indicate low friction loss of smaller pipe due to the short runs. As the pipe selection gets smaller, the velocity increases. The velocity also increases as the pipeline pressure falls. These higher velocities, combined with some thoughtless piping practices by installers or designers not familiar with gas transmission, can lead to some very significant pressure losses and completely upset the system. Often they run for years like this because the maintenance personnel cannot see any damage. For example, the following schematic shows two separate compressor rooms in the same foundry. Both have three 750 cfm @ 100 psig compressors. The piping dictated apparently by the size of the connection is significantly different.

The large reciprocating units were installed in 1968 – 6” discharge to a 10” header with directional angle entry. The velocities are 7.9 fps to 8.4 fps.

6”

6”

6”

2” Discharge Line

2”

2”

2”

3” Header

Three 150-hp Class 750-cfm Rotary Screw

Compressors 1978 units

Three 150-hp Class 750-cfm Reciprocating

Compressors 1968 units

6” Discharge Line

Electric Cost of 18 psig @ .05 kWh / 8760 hrs = over $60,000 /year wasted

The calculated pressure loss is less than 1 psig by the standard pressure chart and the actual measured pressure loss is less than 1 psig. This system is done by the book and works fine. The other system constructed in 1978 (audited in 2001 & corrected) uses 2” lines to a 3” header with “crossing tee” connection.

• The velocities are 69.8 fps to 92.6 fps.

• The calculated pressure loss from standard charts is 8.0 psig -- relatively high, but acceptable at the time.

• Actual measured pressure loss was 18 psig--10 psig of implemented pressure loss was turbulence-driven caused by high velocities and “crossing tees.”


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Typical “Crossing Tee and Dead Head” At higher velocities, these create significant turbulence-driven pressure loss

Directional Angle Entry A proper directional angle entry

1968 to 1978 – What a Difference a Decade Makes! The short cycling created by these high velocities and piping configurations basically helped to cause the plant to run three 150-hp compressors when the original sizing called for two units. At (.05 kWh @ 8760 hours – 176 bhp – this is a waste of over $60,000 /year in electrical energy costs for over 20 years -- $1,200,000!! There are some very distinct guidelines for pipeline velocity, which we have found to work very well in the “real world.”

• Interconnecting piping should never exceed 20 fps. There is no such thing as too low velocity (or too large pipe) for this application.

• Avoid all dead heads, crossing tees, “chokes” for flow meter – use large enough pipe and “long ells.”

• At velocities below 20 fps, design is much less critical.

1” Pipe

3” Pipe

2” Pipe

4” Pipe All pipeline velocities to be 20 fps or less at 100 psig. For Example:

56.8 scfm20 fps

220 scfm20 fps

492 scfm20 fps

838 scfm20 fps

Guidelines for Interconnecting Piping from Compressor(s) through Filters, Dryers, etc. to System

6” Pipe 8” Pipe

10” Pipe 12” Pipe

1901scfm20 fps

5191scfm20 fps

3295 scfm20 fps

7368 scfm20 fps

Use Long Ell’s for turns.

Use 45o directional (to flow) connection for two conjoining

air lines.

Crossing “T”

Dead Head

90o Turns


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The “Hidden Factor” in Poor Interconnecting Piping

LOSS OF EFFECTIVE STORAGE All air systems will do better with storage between the user and the process. Some types of control systems are more sensitive to lack of acceptable minimum storage than others. The amount of effective storage for any control system is really a function of where the operating control band (full load pressure to no load pressure) is equalized by the backpressure in the system.

If the compressor is set to full load at 100 psig and unload at 110 psig and pressure loss in the interconnecting piping and equipment reaches 10 psig at the afterfilter as in the figure at left. The effective storage is 537 gals /71.84 cft. The storage value of the distribution piping is not being utilized. This is much too small to allow efficient operation of the compressor at full r

ange.

he example at right shows

op

ted

lons

500

gallon

Control Band 10 psig

500 cfm 100 hp

66.84 cuft

1 cuft 1 cuft

3 cuft

4” Header 2000 ft

176.5 cuft 1321 gallons 4 psid 4 psid5 psid

EFFECTIVE STORAGE 71.84 cuft / 537 gallons

500

gallon

Control Band 10 psig <1 psid <2 psid

EFFECTIVE STORAGE 248.34 cuft / 1857 gallons

500 cfm 100 hp

66.84 cuft 2 cuft 3 cuft

4” Header 2000 ft

176.5 cuft 1321 gallons

Oversized Heat Sink Cycling

Dryer

Tthe same system set up with a special low pressure drcoalescing filter, oversized heat sink-type refrigeradryer (and good piping). The operating pressure band includes all the headers throughout the system. The system is optimized with regard tocontrols. The effective storage now is 1857 galor 248.34 cft minimum


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Discharge Piping or Interconnecting Piping Here we have more complex considerations: Materials: The discharge air from the compressors can be at 250°F to 350°F (for centrifugal, oil-free rotary screw and reciprocating types), or from 200°F to 220°F (for lubricant-cooled rotary screw compressors), so the pipe must be able to withstand those temperatures. Even if there is an aftercooler that drops the temperature to 100°F, consideration must be given as to the consequences if the aftercooler were to fail. Compressed air-generated condensate tends to be acidic. In oil-free compressors (such as centrifugals and oil-free rotary screws), it is usually very aggressive. The basic objective of the interconnecting piping is to deliver the air to the filter and dryers and then to the production air system with little or no pressure loss, and certainly with little or no self-contamination. Galvanized piping will have the same problems once it begins to peel as we described on the inlet application. In all probability, due to the aggressive acid characteristics of the condensate, the galvanized coating life may be much shorter. Regardless of the “plastic-type” manufacturers claim, we never recommend any plastic-type material for interconnecting and distribution header piping. Most of these materials carry cautions not to be exposed to temperatures over 200°F and to avoid any types of oil or lubricants. Here again, stainless steel or appropriate copper is our number one recommendation for the interconnecting piping from the compressor to the filter/dryers when the compressed air is oil free. It will obviously resist corrosion much better than standard schedule 40 black iron. Some other considerations: Most areas will allow schedule 10 stainless steel in lieu of schedule 40 black iron. For the same diameter pipe, stainless steel or copper will be much lighter and easier to handle, usually lowering the labor cost. For welded connections, stainless steel usually just requires one bead, while black iron pipe usually requires three beads (weld-fill-cover). This should also lower the labor cost. Stainless steel does not usually seal well when threaded. It will do much better with Victaulic-type connections when welding is not practical. Distribution Piping: It is the job of the air distribution piping to deliver compressed air to all parts of the plant with little or no pressure loss. Should certain areas of the plant have surge demands – high flow demand over very short time frames – it also has to handle this, feeding full flow full pressure to that process and not pulling from other processes.


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D

6”

1” 1”1”

2”

1”

1/2”

1/2”

1” Air Line from Compressor Room 2

Air from Compressor Room 2

Air from Compressor Room 1

Dead Head

Drop at bottom of header –should be top

Crossing Tee

All processes total area fed by 1 small 1” drop –

subheader

High Pressure Loss

Tie in at bottom of drop should be -

Old abandoned Orifice Plate Flow meter – 3-5 psid

Air Leak

In this case, the header piping itself is acting as a storage vessel for the process—sometimes a supplemental storage vessel (air receiver) at the right location can be very effective if the headers are too small. This type application has to be evaluated on a case-by-case basis. Sizing Distribution Pipe: Modern thinking would call for 0 psig pressure loss in the distribution by design. The real trick is estimating how much flow capacity is in any one section at the highest possible load. Common Compressed Air Distribution Piping Mistakes

We start with what scenarios the average flow and size for 20 fps or less velocity (although many feel up to 30 fps is all right and actually will work). Then we check for friction loss for the maximum flow at the largest runs. You will have to know or estimate accurately the process demand for this. As stated earlier, oversized header can act as storage and perhaps be the most economical selection. Oversized Header Piping to create effective storage at the process. Often this can work, as show below. In Figure 1, the plant installed a large air receiver with an intermediate controller trying to hold a critical steady pressure at the regulator discharge for optimum production and quality. There was a 200’ run of 4” pipe from the intermediate controller or regulator and the pressure varied from 85 to 90 psig as the regulator opened and closed randomly. The process regulators also delivered erratic pressure to blow off and usually well below the setting. The “pull down band” in the main 4” feed was 4 psig. The regulator feed variance was much greater – some well above 10 psig. The following actions were taken (see Figure 2):

• Move receiver and intermediate controller closer to the process

• Increase main feed line from 4” to 6” to eliminate pressure loss.

• Oversize feeder header to the regulator to 12”. This now created enough storage that regardless of the timing of the regulated “bursts” of air, there was enough storage to feed each line at full capacity without pulling the inlet pressure down at the nearby feeds or even in the overall header.


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3200 GAL

AIR PEELER

AIR PEELER

AIR PEELER

AIR PEELER

AIR PEELER

AIR PEELER

12” HEADER

5” OR 6” STRAIGHT FROM COMPPRESSOR

ROOM

85 PSIG

90 PSIG

72 PSIG

72 PSIG

72 PSIG

72 PSIG

72 PSIG

72 PSIG

Figure 2

AIR PEELER

AIR PEELER

AIR PEELER

AIR PEELER

AIR PEELER

AIR PEELER

4” HEADER

4” FROM COMPRESSOR ROOM MULTIPLE 90O TURNS

85-80 PSIG

59 PSIG

61 PSIG

65 PSIG

64 PSIG

68 PSIG

71 PSIG

Figure 1

Sometimes It Doesn’t Work and You Need the Auxiliary Storage Provided by Properly Sized and Applied Air Receiver

The figure below is a process that feeds a large product collector requiring eight cubic feet of air in .75 seconds every five seconds.

AirPower USA, Inc.

Supply Air CIP Solenoid

Instrument Air

Air to Pulsers 90-50 psig

Regulator

2” 1” 2”

1/2”

1 1/2”

Cashco Regulators Check valve

Supply Feed from System

1”

1 1/2”

Filter Regulator

Filter

105 psig

The main 2” feed from the air system is at 105 psig. The regulators and piping were installed by the contractor. Each time the process runs, pressure quickly pulls down as low as 50 psig, which has very serious negative impact on productivity, product quality, and maintenance of the collector. Regulator and piping sized to surge demands as this must be sized on rate of flow: Establish Rate of Flow: 8 cfm @ 0.75 seconds = 8 x 60 sec ÷ 0.75 = 640 cfm rate of flow

11 www.airpowerusainc.com

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Trying to use the pipe volume 2” and a quick response regulator did not work out: No regulator has quick enough response to handle this 640-scfm rate of flow in 0.75 seconds, so we will supply the demand with storage and allow a metering valve to refill the receiver over a longer time frame (4 seconds), which will reduce the “rate of flow” from the air system significantly. Calculating maximum air receiver pressure loss – 8 cfm demand in 0.75 seconds: TDecay = (Storage Vol) (P2 – P1) . (Net rate of flow) (14.5 psig) 0.0125 min = (V) (10 psig) = 10 V = 116 V = 11.6 cft or 86 gallons (0.75 sec) (640) (14.5) Recommended action to deliver a consistent minimum pressure of 90 psig to the process during pulsing (see figure below):

• Remove 1” piping and both contractor-supplied regulators.

• Install 2” piping to a metering valve (gate valve); adjust valve to refill the receiver in four seconds after the pulser closes. Once set, remove handle to eliminate possibility of inappropriate adjustment.

• Install 120-gallon vertical air receiver to store at 100 psig or more.

Using a standard 120-gallon vertical receiver (16 cu ft), we have the following drop:

Supply Air CIP Solenoid

Instrument Air

Air to Pulsers

Regulator

2” 2”

1/2”

1 1/2”

Check valve

Supply Feed from System

1 1/2”

Filter Regulator

Filter

2”

Metering Valve

2”

120

Gallon 105+ psig

90+ psig

0.125 = 116x = 16 = 7.25 psig Establish new rate for 4-second refill: 0.06 min = (16) (7.25) = 0.966x = (4 sec) x (14.5) 116 = 120 cfm rate of flow Summary: the receiver, along with a metered flow allowing the receiver to refill over four seconds, will reduce the rate of flow from 640 cfm to 120 cfm.

There is not a regulator, which can react in the short amount of time in which the pulsers hit at the required rate of flow. The receiver and metering valve will cushion the distribution piping from this high rate of flow experienced by the system and the 2” main feed, thus allowing it to operate correctly.


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Our piping modification shows a new metering valve sized to handle the new flow at 120 cfm. The bypass is piped with angle entry and exit around the metering valve. The process now operates correctly. What we have had here are two downstream processes that require a little special attention. Until you get into the details, you can’t be sure what the answer is. However, any surge demand should be reviewed as to what effect it is having on the rest of the system and what type of piping design it needs. If the header is sized to 20 fps, not to exceed 30 fps, it will generally not have problems except from these type demands or over very long runs. Loop design systems are always a good idea and often a system that has become restrictive over time may be corrected by creating loops AFTER PROPER INVESTIGATION. There are many other considerations, which are always discussed which we will not cover here – centralized or decentralized system, top or side header connections, pipe slope, etc. Be careful of subheader:

D7

6” Header

1”1” 1”

1”

2”

1”

1/2”

1/2”

WATCH THE SUB-HEADERS

All of these processes fed by ONE – 1” Feed

All of these subheaders are fed by a 1” line. It is always easier to tie in on the end than “hot tap” or put another tap into the main header. Often this can lead to very high pressure loss at the process. Tip: When you build the main header, put in extra taps, valved off on top and sides. Like all things, the header system seems simple until you start looking at what it has to do, what may be required in a “Preliminary Investigation” before actual design.

How Did Your Header Sizing Or Design Get Selected?

Drops to Process/Process Feeds Again, this one seems simple – feed to the process. Just attach to the top or side of the header or subheader – oops! – where does the subheader get its air?


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Size the line size by -- ?? – certainly not opening size! Who in the plant knew the average demand in flow rate? Who knew in rate of flow? What is the minimum required pressure? Why? If you don’t know most of these answers, you are shooting blind. Again, there is no such thing as too big a pipe or hose, unless the size interferes with productivity. Most of the time when we have a low pressure problem because of “being too far from the compressors”, we don’t find the problem in the “main headers” – it is usually in the subheader, feeds, or connections. You must know or estimate the flow demand (and the pressure), then check sizing on all feed lines, hoses, tubes, filter, quick disconnects, regulators, etc. “The Book” talks about allowing from 35 fps to 50 fps. Common sense says “run it as large as you can and when you have to go to smaller line, hose, etc., make it as short as possible.” Look for condensate control at all low and drop points. Some general guidelines to evaluate feeds:

• Know the flow and the optimum entry pressure required

• When required, don’t be afraid to use a needle gauge

• Always measure pressures at load and at rest to accurately see the results

• Know the performance data on all valves, regulators, quick disconnects on the feed lines what effect on pressure drop the anticipated “rate of flow” will have.

Some Common Sense Tips on System Piping

20 or Less Feet per Second Velocity:

Rust, scale, water and oil will not carry down stream and into the element Correct pipe sizing can improve performance and extend filter life. Large water and oil drops and scale will generally travel with the air stream at flow velocities above 30 fps. At velocities lower than 20 fps, they will generally fall out.

Recommendation: Pipe to all filters and dryers with pipe sizing to handle the expected maximum flow at less than 20 fps at pipeline conditions. At these conditions, a “drain leg” ahead of the filter is always a good idea to extend it full performance life and remove any gross load before it reaches the element. Twenty-five to 35 pipe diameters in length of the correct size pipe will effectively slow the air down before it reaches the filter (drain leg). This may improve filter performance and will extend element life.


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Compressed Air Facts That Will Help to Keep the Process Air Clean and Dry Use common sense: Typical application considerations that affect performance.

Many air systems we review with poor air quality problems have ignored some common sense rules that always apply. Keeping these in mind will assure proper performance over a longer period of time at a lower operating cost.

30+ Feet per Second Velocity:Virtually everything such as rust, scale, water and oil will carry down stream and into the element

Three basic rules on controlling water contaminants before and after the dryer, but ahead of a mline filter:

ain

Water vapor will always move from an area of high relative humidity to low relative humidity regardless of the direction of air flow. Don’t ignore leaks.

Water Vapor

Liquid water always drains down by gravity regardless of air flow -- remember when piping and draining. Automatic condensate drains on all risers with “undried air.”

DRY AIR

Liquid water left to stand in air receiver, filter housing, separators, low spots, etc., will evaporate into the dry air raising the relative humidity and pressure dewpoint within the system. Drain condensate immediately and continuously. Example -- Piping to Bagging Unit Regulator:

Original System Connection Corrected Connection Liquid condensate drains into regulator – Liquid condensate now drains from system negatively affects operation and life. pipe and regulator to automatic drain.

No changes to the compressed air system should be made wit r

Regulator gets no liquid.

Hose

Regulator

Condensate

hout permission from the “Compressed Ai

Pipe

Pipe

Regulator

Drain Point

Floor Support

Czar.” No equipment ordered unless the central air utility can handle or has plans to handle. COMPRESSED AIR IS YOUR MOST EXPENSIVE UTILITY


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OTHER GENERAL PIPING & SYSTEM DESIGN GUIDELINES NOT SPECIFICALLY COVERED IN MAIN PAPER

APPENDIX


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compressed air piping selection and design - esl-ie-05!05!10

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