2018 level measurement, part ii - control global

2018 Level Measurement,

Part II

Technology Report

TABLE OF CONTENTSProcess control as inventory control 6

This concept eases troubleshooting, CV-MV pairing, developing models and more.

Prevent pressure transmitter problems 9

Installation details make the difference in DP flow and level applications.

DP depends on good impulses 14

Here are some alternatives in terms of the use of purges and capillary systems.

Polymer reactor level measurement 19

Is it wise or dangerous to back up differential pressure with nuclear instrumentation?

Technology Report: Level 2

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Often, chemical process con-

trol is fundamentally inventory

control. Understanding the

cause-and-effect of the inventory is key

to understanding the process, trouble-

shooting, choosing appropriate controlled

variable-measured variable (CV-MV) pair-

ing, designing cascade or ratio structures,

specifying safety overrides and devel-

oping models (either first-principles or

empirical).

Usually, we don’t measure the inventory.

We measure the response to it. Unfortu-

nately, an initial focus on the response

often distracts the person from seeing the

fundamentals. Recognize the fundamental

mechanism. Here are several examples.

Inventory of gas molecules results

in pressure:

A standard air compressor can be an exam-

ple. The inventory relation of the number of

molecules to the response can be revealed

by the ideal gas law PV = nRT, which

leads to:

P = nRT

V

When you release or add gas, you’re chang-

ing the inventory of molecules, and pressure

is a response. Note that pressure doesn’t

flow in or out. Gas molecules are what

move, and pressure is the response.

Inventory of material results in volume, level

or weight: Consider a flow into and out of

Process control as inventory controlThis concept eases troubleshooting, CV-MV pairing, developing models and more

By R. Russell Rhinehart



a right cylindrical tank of height h and area

A. Again, the relations are simple. Volume is

the product of height and area, and volume

times density is mass: V = hA, Vp = m. So,

the response, level, is dependent on the

inventory of material:

h = m

Ap

To change level, you add or release mate-

rial. Note that level does not flow in or flow

out, but level changes. Material in- and out-

flow causes the inventory to change. Level

is just a measure of inventory.

The situation is similar for solid accumu-

lation. Consider the integrating process

of filling a transport volume with a solids

feeder. The weight is the mass flow rate

times the duration: W = m ∆t.

Or, more properly, with gravity and dimen-

sional units considered:

W = m g gc

= m m ∆t g gc

Inventory of two materials results in com-

position: Consider the continuous blending

of two liquids, A and B. Where x represents

the mass fraction of component A:

xA = mA g

mA + mB

Whether the composition is measured by

volume fraction, mole fraction, normality,

concentration or pH, the measurement is a

response of the inventory of A and B, which

is related to the inflow of each and the

joint outflow.

Inventory of thermal energy (heat) results

in temperature: The relation between heat

added and temperature rise is mCp ∆T = Q,

where ∆T = Tafter- Tbefore heat is added. Then,

T = T0 + Q/m Cp.

Again, the controller does not add tem-

perature, it manages processes that add or

remove thermal energy, and temperature

is the response. It doesn’t matter whether

the heater is a flame, an electrical element

or a chemical or nuclear reaction. It doesn’t

matter whether the object is solid, liquid

or gas. The analysis is similar. If you want

to increase temperature, you don’t add

temperature, you add heat. Temperature is

the response.

Process control is control of the rates that

inventory changes to manage the rates

that P, T, V, h, W and X change. Of course,

there are exceptions—for example, in flow,

we manipulate valve position to increase

or decrease flow rate. Here, I don’t see the

inventory concept that results in flow rate,

so, when we’re introduced to control with

flow rate examples, it might lead one to

miss the inventory concept.

But in most applications, first look for the

inventory, then its response, and control the

inventory.



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Greg: You can’t control something

if you’re not measuring it. There

have been great advancements

in measurement technology. Smart trans-

mitters have increased accuracy an order

of magnitude or more, and drift is so slow

that calibration intervals can be significantly

extended. However, a measurement is only

as good as its installation. Not enough

knowledge is published or presented on

how to make sure the installation doesn’t

limit performance or create maintenance

and reliability issues. Here, Hunter Vegas

and I (co-founders of the ISA Mentor Pro-

gram) offer what we think is important.

The newest resource to our ISA Mentor

Program, Daniel Warren, has stepped up

to offer his personal experiences to help

guide our group. Daniel has more than 35

years of experience as a senior instrument

and electrical design specialist in oil, gas,

chemical, food, mining, utilities, water &

wastewater, and various pulp & paper

facilities, and is the owner of D.M.W Instru-

mentation Consulting Services Ltd.

The most common flow and level mea-

surements use differential pressure (DP)

transmitters with two impulse lines for flow,

and one impulse and an equalization line for

level. Pressure drops are also measured by

a DP with two impulse lines. Many pressures

must also be measured and controlled. Gauge

pressure transmitters vent the low side. Abso-

lute pressure transmitters have the low side

sealed with a full vacuum. Gauge and abso-

lute pressure transmitters (PT) have a single

impulse line. Consequently, a production

unit can have thousands of impulse lines that

often are the weakest link.

Prevent pressure transmitter problemsInstallation details make the difference in DP flow and level applications

By Greg McMillan



The DP and PT installation method and

location should be designed to:

• Prevent a non-representative process vari-

able at the transmitter,

• Prevent extraneous effects at

the transmitter,

• Keep fluid density, composition and

phase the same on both sides of the

DP transmitter,

• Minimize accumulation of solids

and bubbles,

• Minimize plugging, coating, corrosion and

fouling of the impulse lines,

• Minimize time lag(s) from impulse lines to

the transmitter,

• Maximize signal-to-noise ratio, and

• Enable calibration and maintenance of

the transmitter.

The impulse and equalization lines, valves

and manifolds, as well as the transmitter,

all must have wetted surfaces, including

gaskets, O-rings and seals, constructed of

materials that can withstand the worst pro-

cess scenario. This could include corrosion,

temperature swings, sudden pressure and

vacuum swings, mechanical impact (ham-

mering), clean-out procedures, etc.

Let’s first address measurement of gases.

The goal is to ensure that only gases enter

the lines and any liquid drains back into the

process. The transmitter must be mounted

above the process connections with a

uniform slope of at least 1 foot of elevation

change for every 10 feet of length, with

a greater slope generally being advanta-

geous. For horizontal pipelines, the process

connections should be at the top. For ver-

tical pipelines, the process connections

are on the same side as the transmitter. A

vent at the DP transmitter may be useful

for venting the accumulation of low-den-

sity gases (e.g., inerts) and for transmitter

maintenance.

Hunter: Another potential problem with gas

installations is gas condensation. If the boil-

ing point of the gas at maximum operating

pressure is less than ambient temperature,

the gas can condense in the impulse line

and cause intermittent negative pressure

spikes. In this case, the process tubing must

be heat-traced to eliminate this issue. Note

that steam also can condense, but this case

is handled differently. (See steam section

below.)

Daniel: I’ve seen a number of cases where

piping hasn’t been installed adequately to

ensure a sufficient slope for gravity drainage.

I’ve also seen lines that are damaged and

twisted when other mechanical components

are installed as an afterthought. I have “blow-

down” lines installed for gas venting when

isolating and venting a transmitter. This also

gives me a location to tie in a purge to blow

any particulate, oils or condensate back into

the process line.



Greg: When measuring liquids or steam,

you need to ensure that the lines are equal

in length and filled with liquid that has the

same density and no phase changes. The

transmitter must be mounted below the

process connections with a uniform slope of

at least 1 foot in elevation for every 10 feet

of length. Valves at the transmitter should

enable flushing and draining of the lines and

transmitter.

Heat tracing must provide enough heat to

prevent freezing on the coldest day with

the coldest fluid, but doesn’t overheat the

lines and cause flashing (vaporization or

boiling) of the fluid on the hottest day with

the hottest fluid.

Hunter: It’s very important that the tubing

continuously slopes from the process con-

nection to the transmitter. Any high point

along the way can trap vapors and cause

an improper reading. Also, the transmitter

connections usually branch off the main

impulse run. This is done so that if there are

any solids in the impulse line, they’ll drop

into the line section above the blowdown

valves and not impact the pressure mea-

surement at the transmitter.

Daniel: The other thing to take into con-

sideration is the liquid itself. The process

conditions and product will make a differ-

ence in the materials and installation. As an

example, what’s used for water may not be

suitable for liquid natural gas (LNG), dilu-

ent, chlorine, etc. Each of these requires

certain materials for wetted parts (tubing,

diaphragms, O-rings, gaskets, etc.), and it’s

always best to confirm the requirements

with the manufacturers’ tables. The other

thing to consider is the temperature and

the specific gravity. The rangeability as well

as the materials themselves may put a lim-

itation on what can be used to accurately

measure that particular process.

Greg: What more do we need to know

about steam installations?

Hunter: One might consider steam a “gas”

and mount the transmitter above the process

line with a heat-traced line to avoid conden-

sation. However, most transmitters cannot

handle the process temperatures and will

fail in short order. Therefore, a typical steam

installation will mount the transmitter below

the line, let the steam condense, and thus

protect the transmitter from the high tem-

peratures. As long as both legs are equally

filled, the water in the line will not impact the

DP reading, but it will cause an offset for a

pressure transmitter that must be calibrated

out. You also need to freeze-protect the

impulse lines, and keep them warm enough to

avoid freezing but cold enough to ensure the

steam will condense.

Daniel: You don’t have to wait for the

steam to condense to fill the lines during



commissioning. Distilled water can be used

for this purpose. I’ve also used glycol to fill

the lines when setting up transmitters in

cold-climate locations. Seal pots are more

of an old school practice. Their primary use

is to act as a barrier between a harmful pro-

cess, such as a corrosive gas/liquid or steam,

and the transmitter.

The ability to calibrate and maintain the DP

installation generally requires the vent/fill/

flush and drain valves mentioned above,

and a manifold or equivalent piping of

impulse lines that enable the same pressure

to be applied to both sides of the DP for

zeroing. The valves in the lines and mani-

fold must also allow the transmitter to be

safely removed with no exposure to the

process fluid.

Daniel: How you calibrate a transmitter

also depends on how it was installed and

the type (style) of transmitter. I’ve seen a

number of skid-mounted transmitters (and

older installations) that aren’t properly

installed (isolated) to allow for a zero and/

or span adjustment. It’s also easier to do a

bench calibration as compared to a field

calibration. A field calibration can be cum-

bersome, especially if you must have an

assortment of tools and test equipment (air

or nitrogen cylinders, hand pumps, etc.).

Also, testing is limited when you’re dealing

with an older style of DP as compared to the

smart versions.

“One might consider steam a ‘gas’ and mount the transmitter above the process line with a heat-traced line to avoid condensation. However, most transmitters cannot handle the process temperatures and will fail in short order.”



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Greg: Last month, we discussed

a lot of potential problems with

impulse lines. Here, we get into

some alternatives in terms of using purges

and capillary systems, particularly to deal

with problems with solids, polymers, haz-

ardous materials, and corrosive and sticky

fluids that could plug lines, coat and cor-

rode surfaces, or present safety concerns

in terms of maintenance. Hunter Vegas and

I (co-founders of the ISA Mentor Program)

continue this conversation on differential

pressure (DP) and pressure transmitter

(PT) installations.

Some basic requirements are necessary to

prevent the purge flow rate from causing a

variable, significant offset in the measure-

ment due to purge pressure drop or by its

effect on the process. The purge flow must

be constant for all lines. For DP measure-

ments, extra care must be exercised to

ensure that any difference in pressure drop

from purge flow is negligible compared to

the smallest measured DP. Also, the purge

fluid must not appreciably impact the den-

sity of the process fluid or the velocity

profile for DP flow measurements at the

point of measurement.

The purge flow must be visible and adjust-

able. For bubblers, the tip opening should

be increased by cutting an angle off at

the tip after cutting off the tip, so that you

have increased the area of purge flow entry

beyond a simple circular opening. This

should help prevent drying out and plug-

ging at the tip. Loss of purge flow must be

alarmed as an indicator of a plugged point

of entry into the process or insufficient

DP depends on good impulsesHere are some alternatives in terms of the use of purges and capillary systems

By Greg McMillan



purge pressure to prevent backfilling of

the impulse line(s) or bubbler dip tube with

process fluid.

Hunter, what are some of the prob-

lems you’ve seen and what guidance do

you offer?

Hunter: Some issues I’ve seen with purges

and bubblers include:

• Leaks in the purge or tubing—This will

make the measurement read low.

• Sludge in the bottom of the tank or poly-

mer build-up on the bubbler tube—This

will make the measurement read high.

• Tank contents whose specific gravity

changes—This will make the measure-

ment read high or low depending on real

specific gravity versus calibration. Bub-

blers work best on tanks whose contents

are consistent.

Greg: Capillary systems have been used to

eliminate impulse and equalization lines, but

careful attention must be paid to design

and installation. Changes in hydraulic pres-

sure from fill fluid density changes caused

by temperature changes must be mini-

mized. The response should be made as fast

as possible by reducing resistance imposed

by capillary length and size.

The capillary must be secured, and tem-

perature effects from ambient conditions

and proximity to uninsulated equipment or

piping minimized. For DPs, the capillaries

should generally be the same length and

size. If not, the offset imposed in DP calibra-

tion must be constant.

Even when these criteria are seemingly

met, I have seen some strange problems

with capillary systems. One very large plant

extensively used them because of mono-

mers and polymers that could coat and

exhibit a runaway polymerization upon

stagnation. All of the measurements exhib-

ited poor response and accuracy. It was

found that the systems were filled by a ser-

vice company that didn’t have the skills or

equipment to ensure that there were no air

bubbles left after the capillary systems were

evacuated and filled.

In other cases, I have seen sudden move-

ments in pressure and level that are too

fast and uncorrelated with anything in the

process. They were tracked down to unse-

cured capillary systems blowing in the wind.

In another system, there were slow changes

over a period of hours that were tracked

down to changes between sun and shade

by clouds or time of day.

Hunter, what are some mistakes and what

are some best practices that come to mind?

Hunter: Specifying a capillary system is

much more involved than it might look.

There are many issues that should be con-

sidered, including:



• Process conditions—Vacuum or high tem-

perature can boil the fill fluid and damage

the seals. There are fill fluids that can

handle these conditions, but they tend

to be very viscous and slow the pressure

response. Variable process temperatures

can shift the zero as the fill fluid expands

and contracts.

• Ambient conditions—Low ambient tem-

peratures can make viscous fill fluids

respond even slower. Less viscous fluids

can handle lower ambient temperatures,

but generally can’t handle high pro-

cess vacuums or process temperatures.

Also, varying ambient temperatures

can expand and contract the fill fluid in

the capillaries, which will result in zero

shift errors.

• Capillary size—Larger capillaries have

faster pressure response but will be more

severely affected by ambient tempera-

ture variations. Small capillaries will not

be as impacted by ambient temperature

changes but will respond slower, espe-

cially if the fill fluid is more viscous.

• Seal configuration—There are many con-

figurations, including single-seal, balanced

dual seal (two seals the same size with

the same length capillaries), and unbal-

anced (two seals that have either different

seal sizes or different capillary lengths).

Each configuration has pros, cons and

cost differences, and should be evaluated

carefully.

• Electronic, remote DP transmitters—

Essentially two single-seal transmitters

that read the process and calculate the DP

are another option that can make sense in

some applications.

In summary, there are many design

tradeoffs between seal configuration, dia-

phragm size, capillary size and fill fluid type,

and the entire system must be carefully

evaluated to select the proper equipment

for a given application.

Regardless of the seal type chosen, dia-

phragm seals are sensitive and easily

damaged. Many units have been ruined by a

careless mechanic who tossed them on the

ground, installed them without the proper

O-rings or spacers, or tried to clean them

with a wire brush or scraper.

For additional details, see Tip #14, Capillary

System Pitfalls in our ISA book, “101 Tips for

a Successful Automation Career.”

Greg: I looked at a vintage copy of “Chem-

ical Seal Installation & Instructions” from

WIKA, and was impressed by the extensive

guidance and installation details of the

rights and wrongs. Here are some notable

highlights of lowlights (some more obvious

than others).

• Make sure the seal temperature rating

(highest being 225° C) is greater than the

maximum process temperature.

• Do not touch the diaphragm, especially

with any sharp objects.



• Do not place exposed diaphragms on

floors or workbenches.

• For flange-mounted seals, use correct

gaskets and bolt sizes tightened to cor-

rect torque.

• For parallel-threaded-type use suitable

washer, and for tapered threads, use

thread tape or resin compound.

• The instrument must be securely mounted.

• For vacuum or absolute pressure mea-

surement, instrument must be below the

process connection.

• The height of the instrument above or

below the process connection must be

less than 7 m.

• Maximum instrument range must be sig-

nificantly greater than height times fill

specific gravity.

• Do not kink capillary (minimum bend

radius of 5 cm).

• Do not let capillary come into contract

with any equipment or piping.

• Secure and protect capillary.

• Coil extra capillary with minimum radius of

50 cm.

• Do not separate diaphragm from capillary

or from transmitter.

• Return capillary system to supplier if there

are any cracks or damage to seal or capil-

lary.

“For DP measurements, extra care must be exercised to ensure that any difference in pressure drop from purge flow is negligible compared to the smallest measured DP.”



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QOur project involves a reactor that

is 5 m in diameter and about 25

m in height. Presently, its level is

being measured by a differential pressure

(DP) gauge, capillary type, and I was told to

install, in addition, a radiation level detec-

tor. Is the addition of a radiation-type unit

advisable, and what are the advantages and

disadvantages of these two options? Is the

radiation-type a better choice? Can radi-

ation sensors be dangerous if the reactor

walls are thick and hot?

Rahim Salamat, [email protected]

AFor a couple of decades, I was the

chief instrument engineer at C&R, and

during that period, we must have designed

nearly 100 polymer reactor control systems.

So your question is familiar, but it is lacking

the key information: is this a batch or a con-

tinuous reactor?

If it’s a batch reactor and if you have good

flowmeters on the charging side, you

might not measure the level at all, but just

depend on the batch flowmeters for recipe

formulation and add a high-level interlock

for safety. If the accuracy or reliability of

the flowmeters is insufficient, and if the

full weight of the reactor is more than four

times its empty weight, you might put the

reactor on load cells.

To consider nuclear sensors, you need an

NRC license, an on-site certified radiation

officer, and if you have heavy coating, it

will still affect accuracy. In addition, you

must also arrange for source disposal. For

DP, extended diaphragms (Figure 1) with

Polymer reactor level measurementIs it wise or dangerous to back up differential pressure with nuclear instrumentation?



equal-length capillaries and temperature

compensation for ambient temperature and

sun exposure variations can also give rea-

sonable performance.

Although in batch reactors, the residence

time is measured by a timer; in continuous

reactors, residence time is a ratio of react-

ing volume divided by the outflow (V/F),

where V is a function of level. Therefore,

in controlling continuous polymer reac-

tors, level measurement is not optional,

but a must.

Figure 2 illustrates such a control system.

The selection of the level sensor should

consider the comments I made in con-

nection with the batch reactors, and

some people might also consider the use

of self-diagnosing laser (up to 300 °F, if

the transmittance in the vapor space and

the reflectance of the polymer surface is

acceptable) or noncontacting and self-di-

agnosing radar (up to 500 °F, if there is no

coating, condensation or crystallization on

the antenna).

Béla Lipták, [email protected]

AThe biggest reasons not to use radio-

active measurement are:

1. The instrument rays have to shine through

the walls of the reactor so that the

receiver can absorb them, but the radio-

active beam is not concentrated in one

point like a laser, so sometimes you will

have scattering of the radioactive beam,

which could harm personnel.

2. Generally, reactors have extremely

thick walls requiring a very high-energy

source, which, over time, may make the

reactor walls radioactive around the the

beam area.

3. Most radioactive systems need to be

close if not in contact with the surface

of the walls of a hot reactor. This could

damage the source, which could cause

radiation to leak.

Alex (Alejandro) Varga, [email protected]

EXTEND THE SEALSFigure 1: In differential pressure (DP) applica-tions, extended diaphragm seals can prevent plugging, and with equal-length capillaries and temperature compensation for ambient temperature and sun exposure variations, DP can give reasonable performance.

X X XX

X

X

X

XXX

X

Capillary

Filled elements

To controller

Differential pressure transmitter

SetProduction rate

FRC

HIC

FT

LT

FT

FT

M

FRC

FRC

FY

LY

Ratio setting Manual setting of adjustable

residence time (V/F)

Feed A

Feed B

V/F

V

F

SP

Product

X



AI have not come

across DP level used

in polymer reactors. Even

with designs from 40

years ago, we were using

nucleonic/radiation level.

However, that is a small

sample of the total number

of polymer reactors in

the world.

Most of the reactors I

worked with did not use

level. They just batched

in a certain quantity and

called it good. In the

cases I’m familiar with,

there is no real bene-

fit to knowing level in a

batch reactor. Perhaps

most of the modern poly-

mer reactors are now

semi-continuous.

There are regulatory dif-

ficulties involved with

nucleonic installations. In

the jurisdictions I know

about, you have to have

a trained and certified

radiation officer on-site.

There is also a perception

that such devices are very

dangerous and difficult to

monitor/maintain/control.

Simon Lucchini, CFSE, MIEAust

CPEng (Australia)

Chief controls specialist, Fluor

Fellow in Safety Systems

[email protected]

AI lived with this ques-

tion for 15 years. The

only successful approach

was a blow-back dip tube.

This did plug now and then,

and we used a long rod

with a drill bit welded to the

end to clear it. It plugged

because the blow-back air

was supplied at too low a

pressure. The reactor nor-

mally ran at a high vacuum,

X X XX

X

X

X

XXX

X

Capillary

Filled elements

To controller

Differential pressure transmitter

SetProduction rate

FRC

HIC

FT

LT

FT

FT

M

FRC

FRC

FY

LY

Ratio setting Manual setting of adjustable

residence time (V/F)

Feed A

Feed B

V/F

V

F

SP

Product

X

CONTINUOUS REACTOR CONTROLSFigure 2: In continuous reactors, residence time is a ratio of reacting volume divided by the outflow (V/F), where V is a function of level, and maximum production rate equals maxi-mum volume divided by minimum residence time. Therefore, in controlling continuous polymer reactors, level measurement is not optional, but a must.



but we discovered (by watching the oper-

ators late at night) that when the valves

became plugged, the operators would turn

off the vacuum and pressurize the vessel

until the valve plugging cleared. And the

blow-back tube was filled with polymer.

The proper installation sequence would be:

air supply header with filter (not regulator)

> check valve > needle valve > rotameter >

dip tube.

The polymer would not be pushed into the

dip tube because the pressure downstream

of the needle valve would rise to block

backflow. The standard air supply regulator

has downstream pressure protection, and

when the downstream rises the regulator

vents and allows backflow.

It took a long time to discover this. Our

installation did have a radioactive source

in the agitator shaft, and the detector

outside. A “micro-micro-ammeter” ampli-

fied the signal. Most of the time it worked

well. The problem was the way the vari-

ous parts were electrically grounded, so it

appeared to be unreliable. Any welding in

the building caused a panic.

Cullen Langford, [email protected]

“To consider nuclear sensors, you need an NRC license, an on-site certified radiation officer, and if you have heavy coating, it will still affect accuracy. In addition, you must also arrange for source disposal.”



2018 level measurement, part ii - control global

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