© HOERBIGER 2007

Valve Theory and Design

Compressor valves permit one-way

flow of gas into and out of the cylinder.

There must be one or more valves

for suction and discharge in each

compressor cylinder end.

A compressor valve must open and close

with every stroke of the piston – 1000

times or more a minute, and must do

so day after day, month after month. It

must do this directly in the path of the

gas stream which can be entrained with

liquids, foreign particles, corrosive gases

or materials. A valve is also subjected

to all types of destructive forces such as

tension, compression, impact, twisting,

bending, abrasion, erosion and extreme

heat or cold. It is a marvel then, not that

we have valve trouble, but that we have

as little as we do.

A failure of the valve to perform means

the compressor must be shut down for

corrective action. Consequently, anything

that can be done to improve the operation

of a compressor valve will improve the

operation of the overall machine.

A compressor valve requires only three

components to do the job it is required

to do:

� Valve seat

� Sealing element

� A stop to contain the travel of the

sealing element

A valve comprised of the above

components installed in a modern

compressor would not fulfill life and

efficiency requirements. Due to the

high sophistication level of today’s

reciprocating compressors, the demands

on a compressor valve require a much

more elaborate design than the three

basic components mentioned above.

The basic demands on today’s compressor

valves require:

� A large passage area and good

aerodynamics of flow for low

throttling effect (pressure drop)

� The moving parts should be of a low

mass for low impact energy

� A quick response to low differential

pressure

� Small outside dimensions to allow for

low clearance volume

� Low noise level

� High reliability factor and long life

� Ease of maintenance and service

� Tightness in closed position

The valves have the greatest effect on the

operation of a reciprocating compressor

in terms of its efficiency and mechanical

performance.

© HOERBIGER 2007

A compressor valve regulates the

cycle of operation in a compressor

cylinder. Compressor valves are

pressure activated, and their movement

is controlled through the compression

cycle. The valves are opened solely by

the difference in pressure across the

valve; no positive mechanical device is

used.

The compressor valve cycle is best

explained by correlating piston

movement to the pressure volume

diagram.

Gas expansion

At top dead center, the piston stops

and changes direction so piston velocity

is momentarily zero. When the piston

starts moving, the small amount of

gas remaining in the clearance volume

expands. Both suction and discharge

valves are closed as the volume in the

compression chamber increases, cylinder

pressure decreases until it falls slightly

below suction line pressure. The under

pressure in the cylinder permits suction

pressure P1 to push the suction valve

open.

Gas intake (suction event)

Once the suction valve is open, gas from

the suction plenum is drawn into the

cylinder (point 1 in the diagram). As

the piston nears the end of its suction

stroke, its speed decreases which causes

a decrease in the gas velocity through the

open valve. In a properly designed valve,

the spring-load closes the valve at the

moment the piston reaches its bottom

dead center (point 2 in the PV-diagram).

Gas compression

At bottom dead center, the suction

and discharge valves are closed and

the cylinder is full of gas at suction

pressure. The piston motion changes

direction, reducing the volume in the

compression chamber which causes

the gas increase its temperature and

pressure. At p2, discharge pressure, the

pressure in the cylinder is equal to the

pressure in the discharge line. This static

pressure across the valve, keeps the valve

closed. The piston moves to increase

the pressure by the amount sufficient

to open the discharge valve (point 3 in

the PV-diagram). This excess pressure is

necessary to overcome the equalization of

static pressure on the valve plate and to

lift the valve plate, against spring load.

Gas discharge

When the discharge valve opens, the gas

is delivered into the discharge line. As

the piston speed decreases, the velocity

of the gas passing through the valve also

decreases and cylinder pressure drops in

diminishing waves to p2. As the piston

reaches the end of its discharge stroke

(point 4 in the diagram), the discharge

valve is gently lowered onto the valve seat

at top dead center by its springs.

Compression work

The area enclosed by the pressure-volume

curve represents the total work performed

in the compression cycle. The portions

of the area above the discharge pressure

(p2) and below the suction pressure (p1)

indicate the work required to activate the

valves and overcome the flow resistance

in and out of the cylinder.

Fundamentals of the compression cycle

SuccionPienum

DischargePienum Stroke

Mean Piston Speed

s... Stroken...Speed (rpm)

P 2

P 1

FromSuction Line

ToDischarge line

P 1

P 2

4 3

21

Cmax mC = s·n30

Fig. 1. A schematic drawing of a horizontal, single-acting reciprocating compressor (top), piston speed (middle) and cylinder pressure versus volume diagram (bottom).

© HOERBIGER 2007

a (∆p)

Initial Motion Conditions

ccacce

lerati

on

v=ve

locity

s=lift

v,s

t

t

lift=distance from seat

∆p spri

ngloa

d

Once we understand how the pressure

in the compression cycle activates

the valves, we can look closely at the

factors that affect the valve’s operation.

When the valve is closed, part of the valve

plate or valve ring is firmly set against

the seat lands. The sealing element

initially lifts off the seat land slowly but

accelerates rapidly towards the guard

once three forces are overcome:

� cylinder pressure is exposed to the

entire surface area of the sealing

element

� the sticking effect of lubrication or

condensate

� the spring load force (Fig. 3a)All of these factors account for the initial

pressure differential between cylinder and

line pressure at valve opening that is seen

on all PV-diagrams.

Pressure differential opens the valve

To lift the sealing element off the

seat land, a pressure differential is

required across the sealing element. The

difference in area of a sealing element

is normally 15% to sometimes as high

as 30% between exposure underneath

(seat side) and exposure on top (guard

side). Since there is always some leakage

through the closed valve plate along the

seat lands, there is a certain amount of

pressure build-up in this area. Therefore,

the actual pressure differential needed to

break the valve open is only 5% to 15%

over the line pressure.

As the sealing element lifts off the seat

lands, it accelerates rapidly against the

spring load toward the guard. The sealing

element impacts against the guard

causing the opening impact, at this stage

the valve is considered fully open.

Gas velocity keeps the valve open

Piston velocity at top or bottom dead

center is zero and increases gradually to

a maximum at the middle of its stroke.

Valve velocity follows a slower path than

the piston (Fig. 3b).

The flow of the gas out through the seat

keeps the sealing element open. As the

flow diminishes due to the decreasing

piston speed, the springs or other

cushioning elements force the sealing

element to return to the seat lands and

close the valve in time. Preferably, the

valve is completely closed when the

piston is at or near dead center.

Fig. 2. Sealing element movement in suction and discharge valves.

In

Suction Valve

Suction Line Pressure

Flow throughopen suctionvalve to cylinder

Spring

Closed

Open

Out

Discharge Valve

Cylinder Pressure

Flow through opendischarge valveinto discharge line

Disc

Open

Closed

Piston Side

Fundamentals of valve operation

Fig. 3a (top). Linear acceleration to

overcome springload.

Fig. 3b (bottom). Piston velocity versus

sealing element velocity (lift).

© HOERBIGER 2007

In this section we explore the geometry

of a typical plate valve. Other valve

types, like ring valve, poppet valves,

and channel valves can be analyzed

using the methods we use here.

Every valve has three critical areas:

� Seat area

� Guard area

� Lift of gap area (passage area)

The flow of gas through a valve is

demonstrated in the Fig. 4a. The gas

always passes first through the valve

seat, then through the lift area where it

is forced to turn twice and finally through

the valve guard.

From these three areas, the lift area,

which varies with the applied valve lift, is

the most important one. This is why valve

velocities are calculated based on lift

area. However, it is equally important that

the other two areas are within a certain

proportion to the lift area to allow for an

efficient valve.

Seat Area

The seat area is the area where light

can be seen when the valve seat is held

against a light source. This means, that

the webs or bridges on a valve seat are

not included in the seat area. The shaded

areas in the bottom view of a drilled and

milled valve seat are considered seat

areas (Fig. 4b).

In most valves, the seat area is somewhat

smaller than the lift area at maximum lift.

This is tolerated since the flow conditions

through the valve seat are more favorable

than the ones through the lift area.

Guard area

The available guard area is the area that

remains open when the valve plate is

placed on top of the guard (that is, when

the valve is open). Ports or holes that are

covered by the sealing element when the

valve is open are useless.

Valve geometry

Valve Lift (N)

Guard Port (B)

Seat Land (W)

Seat Port (A)

Fig. 5. Cross section of a valve guard (top), seat (bottom) and sealing element (positioned against guard).

BottomView

Gas Flow

Fig. 4a. Gas fow through an open valve.

Fig. 4b. Valve seat area.

© HOERBIGER 2007

Valve lift

The lift area is generally the area of

reference in a compressor valve. It is

the sum of all circumferences of the

small diameters where the flow turns

multiplied by the valve lift.

Based on the foregoing equation, it

appears to be advantageous to design a

valve with a large number of narrow ports.

This approach has its limitations since a

certain ratio between the seat and guard

areas, respectively, to the lift area must

be maintained.

Valve lift

Valve lift is governed by the goal to

design valves with acceptable life and

uninterrupted service. Since the plate

or sealing element opens and closes

with every revolution of the crankshaft,

such factors as rotating speed, operating

pressure and molecular weight of the gas

determine the limits of allowable valve

lift. The impact resilience of various

materials used for valve plates (steel,

polymers, etc.) also has an influence on

maximum acceptable valve lift.

Different valve manufacturers will use

more or less conservative guidelines for

allowable lift for a given set of operation

conditions.

Excessive valve lift can have detrimental

effects on valve life, due to high-velocity

impact forces, valve flutter, late closing,

and other life-deteriorating developments.

Once an acceptable valve lift is defined,

the rest of the valve geometry can be

selected to balance the ratios of seat and

guard area to free lift area.

The diverse applications results in a

variety of valve concepts. For example,

slow-speed applications favor wide-ported

seats and guards and high valve lifts,

while high-speed applications, require

narrow ports and lower lifts would be

applied.

H

d 6

d 2

d 4

d 5

d 3

d 1

d 7

Fig. 6. Cross section of a half valve guard (top), seat (bottom) and sealing element (positioned against guard). This diagram illustrates the dimensions used to calculate the passage area.

© HOERBIGER 2007

Compressor valve performance is

generally categorized in two ways,

efficiency and life. Valve efficiency

can be defined as that part of the

overall compression horsepower that is

attributed to the accumulative flow loss

in all valves.

Measuring flow loss

Each valve causes a certain pressure

loss as gas flows through it. The factors

that determine this loss are based on the

valve’s geometry, valve lift, free lift area,

and spring load.

Analyzing one cylinder at a time, we can

calculate the flow losses for both suction

and discharge valves and determine the

horsepower required to flow gas past the

valve. We can measure this pressure drop

by conducting a static flow test.

Calculating the coefficient of flow

The purpose of static flow testing is

to measure the pressure drop through

the valve. The pressure drop is used to

calculate the coefficient of flow that

corresponds to the lift area.

The valve is mounted in a sufficiently

sized pipe and a constant flow of air

passes through the valve. The pressure

drop across the valve is measured.

The coefficient of flow is a volumetric

flow rate (or capacity). It can be

determined from the measured pressure

drop, the measured flow rate, and the

density of the gas. The coefficient of flow

is a parameter for the flow efficiency

of a specific valve and can be used to

calculate valve losses in a compressor

cylinder with some degree of accuracy.

The derivative of the coefficient of flow

is called the equivalent area or phi-value.

It constitutes an orifice area (with a

flow coefficient = 1) with an equal flow

restriction as the value with its multiple

port openings.2

=

AreaEquivalentAreaLiftFlowoftCoefficien

The coefficient of flow and equivalent

area are defined for a specific valve

geometry at a specific valve lift and vary

as these criteria are changed.

Pressure drop

Valve

�p

Flow

Fig. 7. A static flow test measures the pressure drop through a valve. The pressure drop

is used to calculate the flow coefficient and equivalent area.

© HOERBIGER 2007

The suction and discharge valves are

designed differently because there are

differences in

� the piston velocities at which suction

and discharge valves open and close

� the duration of the suction and

discharge events, and

� the pressure and temperature at

which suction and discharge occur

All gas displaced in a compressor cylinder

must pass through the suction and

discharge valves mounted in the cylinder.

For efficiency reasons, it is important

that the gas passes through the valves at

a reasonably slow speed. This speed is

referred to as either the mean gas velocity

or valve velocity. High mean gas velocities

have the following consequences:

� large pressure loss due to the

throttling effect within the valve

� temperature increase due to gas

friction

The pressure loss results in higher

horsepower consumption.

Higher intake temperature means a lower

actual output since less gas is taken into

the cylinder.

The calculation for mean valve velocity

is given in the side bar. Note as lift

area increases, the mean gas velocity

decreases.

Mean gas velocity

Valve velocity

The valve velocity is calculated at

the lift area and decreases if the

lift area is increased. The mean gas

velocity through a valve is a widely

used parameter for comparative valve

evalution.

where:

vm is the mean valve velocity [ft/min]

(m/sec)

d is the piston diameter [inch] (mm)

s is the piston stroke [inch] (mm)

n is the compressor speed in RPM

π is 3,14159

fe is the lift area of valve [sq. inch]

(cm²)

z is the number of suction or

discharge valves per cylinder end

k is the [24] for inches (1.2 x 107) for

S.I. dimensions

Typical valve velocities

� Slow-speed transmission line

2,500-4,000 ft/min (12-20 m/s)

� Process and petrochemical

3,000-6,000 ft/min (15-30 m/s)

� High-speed gas gathering

5,000-11,000 ft/min (25-55 m/s)

� Air compressors

8,000-13,000 ft/min (40-65 m/s)

© HOERBIGER 2007

The seat area, free lift area, and guard

area have little meaning individually,

but all three together determine the flow

losses through a valve.

To measure or calculate flow loss, various

indicators have been introduced and

are used today by valve manufacturers.

Typical indicators are:

� coefficient of flow, and/or

� equivalent area

The resistance factor, used by some

manufacturers, relates the equivalent area

to the available pocket area.

The valve geometry and comparative

performance parameters, discussed in

this section, serve a purpose in comparing

competitive products and can be valuable

in deciding which might best be applied

in a certain operating environment.

The rated mileage of a car, for example,

will differs from the actual gas mileage

achieved since gas mileage varies with

the driving pattern of the operator.

The same is true for valves, where the

valve dynamics and motion pattern

of the opening and closing cycle of a

valve can drastically influence the valve

performance, both in terms of efficiency

and life.

Flow loss

Valve motion

Fig. 8. shows a pressure-volume curve for

a typical compression cycle and above

and below are the motion patterns of

suction and discharge valves.

Notice that the discharge valve must open

and close in a shorter time and under

more severe pressure and temperature

conditions than the suction valve.

Rarely do valves perform in such ideal

motion patterns as shown. Influences

such as the valve geometry, spring load,

pulsations, oil sticking, and uneven flow

due to pocket configuration affect valve

motion, and not all of these factors can

be quantified or easily predicted.

Fig. 8. A typical compression cycle.

1 4

32

P2

P1

TDC BDC

DischargePressure

Seat

Guard

SuctionPressure

Guard

Seat

valv

e lif

tva

lve

lift

piston displacement(stroke)clearance

discharge valvemotion

suction valvemotion

The motion pattern of a suction or

discharge valve is essentially the plot

of its opening and closing path. Ideally,

every valve should open early on with a

minimum of differential pressure between

cylinder and line pressure; it should

open quickly, but at the end of its lift,

it should impact against the guard with

low-impact velocity. During the discharge

or inlet cycle, the valve should remain

fully open without flutter, and towards the

end of the stroke it must close in a timely

fashion and do it with low-impact velocity

when striking the seat face.

Plate valves have only one moving part.

In plate valves, where there is only one

sealing element, the motion is easier to

control and predict than in valves using

multi-sealing elements such as multi-ring

valves or poppet valves.

Actual valve motion behavior has been

measured and analyzed by HOERBIGER

for more than 35 years. Such studies are,

however, limited to laboratory or in-house

tests and remain, so far, too costly and

cumbersome to do in field units.

Examples of poor dynamics

Figures 9a and 9b are valve motion

diagrams that show flutter (9a) and

delayed closing (9b)

Valve flutter is not only detrimental to

valve life because of multi impacting, it

reduces the effective lift area and also

flow efficiency.

Delayed closing is especially damaging

to valve life since it is associated with

slamming of the valve against a seat;

the resultant back flow lowers overall

efficiency by a substantial margin.

Credible valve manufacturers for some

time have used valve motion studies to

improve valve performance and will alter

design conditions of the valve offered

for a specific application to optimize

performance.

Crank Angle

Valv

e Li

ft

TDC

Guard

Seat

Guard

Seat

Crank Angle

Valv

e Li

ft

TDC

Fig. 9a. Valve flutter. Notice how the valve lift fluctuates between the seat and the guard.

Fig. 9b. Delayed valve closure. Notice how the valve closes well after top dead center.

© HOERBIGER 2007

Valve dynamics

Engineering valves that perform

Computer Modeling

Computer modeling simulates valve

motion and is a tool valve designers

use to predict valve behavior during the

design phase.

The objective is to design valves in a

geometry with a lift and springload that

provide a motion pattern as close to ideal

as possible.

If adverse effects such as flutter or early

or late closing can be prevented, both

valve efficiency and valve life will be

substantially improved.

Engineering know-how and support on

all levels

Compressor valve engineering and

manufacturing may seem simple on the

surface. However, designing a long-

lasting, efficient valve requires an entire

team of specialists in the fields of:

� metallurgy

� flow dynamics

� non-metallic material specialists

� thermodynamic properties of gas

� mechanical engineering

� compressor applications

� 3D modelling

Once a design is developed, the valve

needs to be manufactured in the most

cost-effective manner using the best

choice of materials. That requires an

entirely different team of specialists:

� procurement

� manufacturing design engineers

� CNC programmers

� experienced machinists

� quality control

� field engineers

Because the valve is such a critical

component in every compressor, users are

ill-advised if they accept compromises in

design and quality.

The best valve performance in both life

and efficiency is normally found from

designers who provide the engineering

know-how and support on all levels, as

outlined above.

Although the reciprocating compressor

industry is considered mature,

reciprocating compressors continue to be

valuable plant assets because they are

easy to repair and they run reliably.

Older field units or installations where

performance is not up to expectations

often can be corrected or upgraded to

better designs, and the money invested

frequently experiences a fast payback.

Therefore, in an effort to keep these

assets efficient and up-to-date, valve

technology and research has advanced

continually in the last 25 years.

The introduction of non-metallic valve

sealing elements, for example, in the

mid-1970s, allowed valve designers to

invent valves that could withstand higher

impacts for longer periods of time and

wouldn’t damage the compressor cylinder

if they failed (like steel plates can).

New valve designs lead to new

compressor designs—ones that could

operate at higher speeds.

Developments in electronics lead to more

responsive, more automated capacity

control devices.

Research and development continues

to advance the field of valve design and

will undoubtedly continue to bring about

new valve concepts that will ultimately

improve new cylinder designs.

© HOERBIGER 2007

Valve upgrade and retrofit proposals

A valve retrofit proposal starts with a

study of the operating conditions and a

list of the customer’s expected objectives.

Proper material selection is critical to

a valve’s performance. Some materials

perform good at low temperatures but

become brittle at higher temperatures,

some resist corrosive environments better

than others, some are designed for use

with specific gases. To select the right

material and components for the valve, a

valve application engineer must study the

gas analysis and operating pressures and

temperatures.

The application may contain clean, dry

gas, or it may contain contaminants. If

contaminants or impurities cannot be

removed the valve design must minimize

their negative effects. A thermodynamic

study tells the valve designer if the

compressor will operating close to the

gas dew point (the point where the gas

changes to a liquid).

The compressor design is also important.

Is the compressor lubricated? is it a

multi-stage unit? How are the valves

positioned in the cylinder? Do the

passages allow for even the gas flow

distribution in the valve cage?

Similarly, fluctuations in pressures or

molecular weight of the gas or changes

in the operating mode, such as added

clearances, should be known when

designing a replacement valve so the

“window of proper application” can be

selected for a successful retrofit.

Correct dimensional information of the

envelope of the current valve is also

required.

The more the designer knows about

the application and the expected

performance, the more successful the

valve retrofit will be.

What your proposal should contain

Besides price and delivery, look for the

following information in your valve offer:

Fundamental Valve Information

� valve geometry

� valve lift

� the mean gas velocity calculated at

medium piston speed

� the selected materials for the

different valve components

Valve Performance

Some valve manufacturers will provide

more comprehensive information on valve

performance such as:

� energy consumption in both absolute

horsepower or percent of adiabatic

� the coefficient of flow for a specific

valve at a specified valve lift or

equivalent area

� the temperature increase generated

through the throttling effect of the

valves. The temperature increases

through valve throttling increases the

discharge temperature and reduces

the cylinder capacity.

� the valve clearance volume for both

suction and discharge because

changes in clearance will affect

volumetric efficiency and may,

therefore, impact on the overall

performance of the compressor.

� In critical applications, it is advisable

to complement static flow criteria

with a dynamic performance analysis

Simulated Valve Dynamics

The most comprehensive analysis

would include valve information, valve

performance, plus a dynamic simulation.

Today, most valve manufacturers have

computer programs that allow them to

perform valve dynamics; and although

there is always a discrepancy between

the computer simulated dynamics and

the real world operation, the proximity of

the two in actual comparisons has been

quite close. Simulated valve dynamics

is used more in valve failure diagnostics

to analyze the performance of a field

installation that is unsatisfactory.

Comprehensive information is required to

run a proper valve simulation and many

customers are not always able to provide

enough information to run the simulation.

With the multitude of brands and valve

concepts that are available, a buyer truly

has a choice. No single valve design will

outperform all others; nor does one valve

cover all the different applications.

Luckily, purchasing decisions can

be made with a good certainty of

performance based on the analytical tools

that are available.

© HOERBIGER 2007

The HOERBIGER Group

HOERBIGER Compression Technology is a business unit of HOERBIGER Holding AG, Zug / Switzerland. HOERBIGER is active throughout the world as a leading player in the fields of compression technology, automation technology and drive technology. In 2008, its 6,300 employees achieved sales of around 1 billion Euro. The focal points of its business activities include key components and services for compressors, engines and turbomachines, hydraulic systems and piezo technology for vehicles and machine tools, as well as components and systems for shift and clutch operations in vehicle drive trains of all kinds. Through innovations in attractive technological niche markets, the HOERBIGER Group sets standards and delivers cutting-edge solutions for the benefit of its customers.

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