A short note on Control Valves
Introduction
A control valve is used to control a physical parameter like level, pressure, temperature or flow to a desired value in the process to ensure quality of end product. it does so by varying the opening of a closure member like a plug over seat or a plug covering or uncovering holes in a cage. Therefore, a control valve can be on-off or throttling (Throttling means vary the flow over a range between fully open and fully closed). A simple scenario of a liquid level control is shown here. A sample picture of control valve showing its various parts is shown here. This note is made to link the various sources of information to one place for ready reference and is in no means exhaustive. However, it would provide lot of required information from a beginner's point of view, like, say, someone who is seeing a control valve for first time and instead of getting lost in exhaustive documentation up front while missing host of important topics, this would help to get a broad overview.
1. what are the main parts of a control valve?
The valve can be divided mainly into three parts (1) Actuator (2) Bonnet (3) Body. There are other parts within these three main areas for proper functioning. A complete dis-assembly of the control valve can be seen in this file. A Bonnet Extension is used in high temperature service to minimize the impact on packing.
2. How can control valves be classified?
There are number of ways in which the valves can be classified. The apex classification is whether the valve is linear or rotary. The major difference between rotary and linear control valves is given in pg # 16 of Ref #1 below. The valve capacity or Cv is higher in rotary and provides better shutoff. However, for throttling cases, linear valves are suitable. It also discusses the situation in which rotary control valves to be used in throttling. Click here to see some classifications. (Ref #1, pg 11)
- Failure of Actuating Energy Source
At the very basic, a valve can be classified based on failure of the actuating energy source. we can have fail close, fail open , fail in place and fail safe. Fail in place is the condition where valve stays in the last position when actuating energy source fails. Fail safe can be configured as fail open, close or fail in place using auxiliary mechanisms.
Flow Characteristics
It can be classified based on its flow characteristics. A flow characteristic is the relation between flow rate through the valve and the valve travel as the travel is varied from 0 to 100%. Four types of characteristic are linear, equal percentage, parabolic & quick opening. which again depends on the type of 'trim' used. The choice between them depends on control valve selection and is presented in following pages.
- Based on flow direction
Usually you have a valve with globe valve body where flow is in up direction, to move from below the port to come out from above the port, this ‘flow-up’ direction has the tendency to open the plug, so it can be called as ‘flow to open’ or FTO valve. Opposite case is ‘flow down’ direction where the incoming flow comes into the port and goes out below the port, this has the action to close the plug, so it can be called as ‘flow to close or FTC valve. Click here for a picture. Usually, one finds only Flow up or FTO valves. Flow down valves or FTC valves used where the design predicts damage due to cavitation and hence anti-cavitation trims are used. The flow direction is marked on valve body with an arrow.
- Direct acting or Reverse Acting
It can be classified on basis of how the closure member acts in the direction of moving stem in case of linear valves. Direct acting means, the valve closes while stem is moving into the body. Reverse acting means the valve opens when the stem is moving into the body. Most of the control valves commonly found are direct acting.
- Classification based on Guidance
Valves can be classified based on how the closure member (plug) is guided in achieving the control. There are mainly three types:-
(1) Stem Guided
(2) port (seat) guided
(3) Cage guided
Guidance in control valve roughly means which member ‘holds’ the stem in place to achieve control.
- Stem guiding:
Therefore, a stem guided valve will have bushing in packing assembly which will hold the stem in place. If bushing is in bonnet, it is top guided valve (also called post guided valve), if bushing is present in body apart from bonnet, then it is top and bottom guided valve. Top and bottom guided valves are usually double ported valves having two ports and two plugs.
- Port (Seat) guiding
The plug has unusual shape with wings or skirts such that it projects into the seat as shown in figure here. This implies stem can be made smaller in diameter than if the valve trim were stem guided thus minimizing sliding friction.
The seat can also be modified to alter the flow characteristic apart from guiding as shown in this figure. Notice in this figure that all the three plugs are identical and the seat ring design is changing the flow characteristic.
- Cage guiding
The cage has ports cast or machined into it. The cage guided control valves throttle flow by covering or uncovering more or less of this port area as plug moves up and down. The cut-away figure shows the use of a cage.
From guidance point of view, the cage and plug are almost tight fit thereby holding and positioning the stem better and hence stem diameter can be reduced than if the valve were stem guided.
Another advantage is if the flow characteristics need to be changed, the same can be done by changing the port shape and size by replacing the cage instead of replacing the plug or seat. This figure shows the design of linear, equal percentage and quick opening designs.
Yet another advantage is that in case of cage guiding, the plug can be made ‘Balanced’. A balanced plug is one which has a through hole drilled from top to bottom of plug. This has the effect of balancing the process pressure on the top and bottom of the plug thereby, the actuator can be made smaller so that it does not have to be large to overcome process pressures when moving the stem. The only disadvantage of using balanced plug is the increased chance of leakage.
3. What are different types of plugs used?
(1) Balanced and Unbalanced
The same is explained above in cage guiding. Click here for a pic of balanced plug
(2) Fluted
Fluted plugs are used to control low flow rates as shown in the figure
(3)Linear, Equal % & quick opening plugs
The figure shows the three types of plugs
4. What are double ported valves and their advantage and disadvantage?
Double ported valves have two plugs and matching pair of seats to throttle fluid flow as shown in this figure. The advantage of double ported valve is that the force applied by process fluid on the stem is minimum because the force acts in the opposite direction on the two plugs as shown in figure and hence the actuator can be smaller.
Disadvantage of double port valve is more seat leakage because of double seat.
5. What is valve packing?
Valve packing is used to prevent the ingress of process fluid into the valve bonnet and at the same time allowing some external device (actuator) to move the stem. Valve packing is shown in this figure
Packing can be live loaded (with spring) or Jam packing (stationary loading) (without spring). In jam packing, the lantern ring as shown in figure above is used as a spacer between two sets of packing rings. It is made of metal and allows lubricant to enter into both sets of packing rings. As shown in the figure, the packing flange is tightened to apply compressive force on the packing follower and the packing rings. It should not be overtightned. Over tightening increases the packing friction for smooth stem movement and also create undue wear on packing leading to leakage.
5.1 Live Loaded packing:
Live loaded packing is insertion of spring in the packing assembly so that elasticity of the spring maintains the proper packing stress as the packing material ages. coil spring is used in place of lantern ring, else Beelleville spring washers are used, then lantern ring is present as shown in figure here.
Packing rings are made of PTFE or graphite. PTFE is used in limited temperature range but has stability against wide variety of chemicals. PTFE is not very tolerant of nuclear radiation. Graphite has far greater ability to withstand nuclear radiation but has more stem friction than Teflon. Another disadvantage is galvanic corrosion in stem and body materials due to its conduction.
Click here to listen to an actual control valve noise due to a faulty packing. The cage and stem assembly was turning due to process fluid force due to insufficient grip by the aging loose packing rings. This problem came about when flow rate was increased through valve. At low flow rate this problem was not there. Replacing the packing assembly solved the noise problem at high flow rates.
Valve packing is explained here in section 27.4, page 1961 to 1969 of Ref # 2
6. What are seat leakage classes?
Valves are meant to allow flow or completely shut off the flow. The complete shut-off however is not absolute. The shut off class therefore, indicates the figure of merit. Six classes are defined using roman numerals
- What is Cavitation , Flashing and choked Flow?
7.1 Flashing
When fluid passes through constrictive passage of a control valve, its velocity increases and according to the equation of continuity,its pressure decreases. This reduction of pressure mostly happens in the valve trim and this minimum pressure is called vena contracta pressure and denoted by Pvc. If Pvc is less than the vapour pressure of the fluid, then the liquid boils and vapours are formed. Shown in this figure. The damage due to flashing is shown here
The point of vena contracta is the point where flashing occurs, thus flashing can damage the trim material like the plug by scoring the surface.
One parameter which is important w.r.t flashing is the Liquid Pressure recovery factor (Fl)
Fl= SQRT ((P1-P2)/(P1-Pvc) )
7.2 Cavitation
Cavitation is the next phase of flashing. After the pressure reduction in trim, the pressure starts recovering as it passes through the trim and into the body of the valve, the vapours generated start recondensing back to liquid once the pressure is more than Pvc. This recondensation happens in the form of a jet of liquid which hits with enormous force on the valve material and can cause deep pits on the surface. While flashing damage is smooth across a surface, cavitation damage is like a pock mark or a pit on the surface. The cavitation damage can be seen in this figure
Flashing & cavitation are explained nicely with pictures in ref # 2 (page 2069 onwards) and occur only in liquid service.
7.3 Choked Flow
Both gas and liquid service experience choked flow. Choked flow is the condition when reducing the downstream pressure does not increase the flow rate substantially as downstream pressure is reduced. This phenomenon can be seen in this graph
An approximate position of choked flow is when the vena contracta pressure is less than one half the inlet pressure,then choked flow is guaranteed.
The valve equation in which the flow rate is directly proportional to square root of differential pressure does not hold in the choked condition.
Note that the choked flow is with respect to downstream pressure only. It does not mean that the flow rate of the valve can’t be increased beyond choked flow. If upstream pressure is increased, the flow rate can be increased.
In gas control valve, choking occurs when the velocity of gas approaches the velocity of sound in that gas which is referred to as critical or sonic flow. The relation between speed of sound and choking is explained very well in page # 2080 of ref # 2.
In liquid service, choking can occur once flashing starts.
8. How are valves selected based on flow characteristics?
The common flow characteristics are Linear, Equal Percentage & Quick Opening. These flow characteristics are shown in this figure.
When to use Linear or quick opening or equal percentage flow characteristics.
Linear: A linear flow characteristic is what it means, the percentage increments in valve stem motion cause corresponding increment to the valve flow capacity. Therefore, if stem moves up by 10%, the Valve flow capacity is 10% of rated Cv.
Equal Percentage:
In Equal %, for equal increments of stem travel, the percentage change in Cv will be same. This is explained very nicely in pg # 23 of Reference #1 below. Click here to see. As can be seen it is an exponential curve, therefore, the flow is less at initial stem travel. 50% of rated Cv is achieved at 87% stem travel.
Quick Opening:
Quick opening is the mirror image of equal percentage characteristics. The valve allows large flows at small increase in stem travel and saturates thereafter.
9. Control valve Selection and sizing basis?
According to ISA S75.01, “Flow equations for sizing control valves”,
9.1 What is Valve Cv?
In simple terms, the valve Cv can be described as no. Of gallons per minute of water at 60 degree F (15.5 deg C) that can pass through a valve with an associated pressure drop of 1 psi.
The Cv of a valve obviously changes with the opening with maximum Cv at 100% opening. For any other fluid, at any other pressure drop, the CV can be calculated using the following formula and test arrangement
ISA 75.01.01-2007 defines Cv for Turbulent flow (When Reynolds number inside the valve is greater than 10,000) as a standard equation
Cv= Qv * sqrt (SG/Delta P)
Where, Qv is the flowrate in Gallons per minute (gpm) at Delta P pressure drop in psi and SG is the specific gravity of the fluid. This also assumes that the valve the full bore wrt to the pipeline.
Another unit Kv is also used which is same as Cv but units in metric system. Cv=1.16 Kv
Also, if the units of Qv is in m3/h and DP is in bar, then Qv needs to be divided by 0.865 to get the proper Cv value
9.1.1 Standard Conditions
The arrangement shown in the figure (Ref # 6, page # 6 onwards) can be used to measure the Cv. The test setup assumes that:-
- fluid flow is flow in turbulent condition;no cavitation and vaporisation phenomena;valve diameter equal to pipe diameter;static pressure drop measured between upstream and downstream pressure taps located as in Figure
- straight pipe lengths upstream and downstream of the valve as per Figure; Newtonian fluid.
9.1.2 Real conditions
Even if the flow is turbulent, The Cv equation is different for compressible fluids and in-compressible fluids and when the flow is normal, critical or reaches the limit (choked flow) and when there is a pipe reducer between the valve and pipe (Piping factor Fp and Flp). These conditions and respective Cv equations are given in page 9 of Ref #6.
9.2 Cv for Non-Turbulent flow (Laminar flow)
Reynolds number is the ratio between mass forces and viscous forces For cases with Reynolds number is low, the fluids are more viscous and equations for turbulent flow need to be corrected by an Fr coefficient. Fr becomes a fundamental parameter to size control valves with low flow where Cv = 1 gpm
These equations are described in Pg # 12 of Ref # 6
The equations above are based on IEC 60534-2-1 and IEC 60534-2-3 standards
The valve Cv should meet the following:-
- An equal % trim should operate below 95% travel at maximum flow.
- A linear and quick opening trim should operate below 90% travel at maximum flow.
Valve Cv was dealt in detail in section 7.
9.2 Body Size
VALVE calculations (pg 17 of basic operation of control valves,ref# 1 and pg 123 of Ref #4, control valve handbook of Emerson Process mngmt)
(1) valve body size, Port size & Diaphragm area
Three terms are introduced here, ∆p(sizing) , ∆p (Throttling) and ∆p (Shutoff)
∆p (Throttling) is the DP across the valve during various stem position
∆p(sizing) is the Dp across the valve at full open condition (in case of ATC-FO valve)
∆p (Shutoff) is the Dp across the valve at full close condition (in case of ATC-FO valve)
∆p (Throttling)= P1-P2 , where P1 is the upstream pressure and P2 is the downstream pressure
Also note, P1 is always greater than P2 . P1> P2
Sizing of valve orifice
∆p(sizing)= P1(min)-P2 (Max) , this is the minimum DP which can be used to design the orifice size for maximum flow rate.
Sizing of valve actuator
∆p (Shutoff)= P1(max)-P2(min) or ∆p (Shutoff)= P1(max) as P2(min)=0.
The maximum shutoff pressure for which the actuator has to force balance to keep the valve shut, and hence the area of the actuator can be calculated.
The actuator diaphragm area calculation is shown in page no. 47 of Ref # 1.
9.3 Coefficients for avoiding valve problems
9.3.1 Cavitation Index Kc
The cavitation index (Kc) of the valve helps in identifying the exact Differential pressure at which cavitation occurs. These equations are from Ref # 4, pg 15
If ∆p/(p1-Pv) <Kc , No cavitation occurs
If ∆p/(p1-Pv) > = Kc, cavitation begins
If ∆p/(p1-Pv) > = ((Kc + Fl)^2)/2 , Pitting occurs
If ∆p/(p1-Pv) > 1, Flashing occurs
From pg # 15 of ref # 4
9.3.2 Choking
If ∆p/(p1-0.7*Pv) > = Fl^2, then choking occurs
From pg # 16 of ref # 4
- Any Sample sizing exercise?
This document can help size a valve for a given condition. Some solved examples are provided for practical sizing of liquid and gas service. See page no 10 onwards (Emerson valve sizing calculations)
11. What are the terms found in the datasheet?
A sample datasheet according to ISA 20.5 is given here
Some of the important terms to understand are Cv, Kv, Fp, P1 (Inlet Pressure), P2 (outlet Pressure), Dp, Pv (Vapour Pressure),Q (Flow rate) Pipeline Size in, Pipeline size out,Fl (Liquid Pressure recovery factor), Bench Range, Leakage class
Important terms found in datasheet are also described in Pg 13 of ref # 6
References
- Basic operation and Function of Control Valves by Cashco
- Lessons in Industrial Instrumentation by Tony.R.Kuphaldt (pg 1943 to 2095) (very good manual with lot of pictures and practical explanation)
- Control Valve Handbook Fourth Ed, by Emerson Process Management
- FCRI course note on control valve. (Very precise & concise note on valve selection and sizing)
- Control Valve technical specification by Control Components Inc (This contains extracts of ISA guide “Control Valves-Practical guides for measurement & control edited by Guy Borden jr & Paul G Friedmann. Provides the technical specification as defined by ISA and mention of relevant standards and specification sheets.)
- Control valve sizing by Parcol Technical Bulletin 1-I (very easy to understand with lot of CFM simulation figures and data)
Relevant Standards:
(1) Control valve terminology as per ISA -S75.05
(2) ANSI/ISA S75.01 for valve sizing (harmonized with IEC standards 534-2-1(incompressible fluids) & 534-2-2(compressible fluids))
(3) ISA S20.50 Specification Forms for Process Measurement and Control Instruments, Primary Elements and Control Valves
(4) ISA S75.11 Inherent Flow Characteristic and Rangeability of Control Valves
(5) ISA SP75.17 Control Valve Aerodynamic Noise Prediction
(6) ISA RP75.23 Considerations for Evaluating Control Valve Cavitation
(7) Fluid Control institute FCI 70-2 Control Valve Seat Leakage
(8) International Electro technical commission IEC 534-8-3 Control Valve Aerodynamic Noise Prediction Method
(9) Manufacturer’s Standardization Society MSS-SP-61 Pressure Testing of Steel Valves
(10) IEC 60534-2-1, Industrial process control valves – Flow capacity – Sizing under installed conditions
(11) IEC 60534-2-3, Industrial process control valves – Flow capacity – Test procedures
(12) IEC 60534-7, Industrial process control valves – Control valve data sheet
(13) IEC 60534-8-2, Industrial process control valves – Noise considerations – Laboratory measurement of noise generated by hydrodynamic flow through control valves
