Topic 7 Control Valves

What We Will Cover Topic 1 Introduction To Process Control Topic 2 Introduction To Process Dynamics Topic 3 Plant Testing And Data Analysis Topic 5 Enhanced Regulatory Control Strategies Topic 7 Process Control Hardware Systems Topic 4 Controller Actions And Tuning Topic 7 Control Valves Topic 8 Process Control Troubleshooting

Control Valves  Control valve construction  Valve types  Control valve characteristics  Valve failure characteristics  Control valve sizing  Control valve performance –Common problems –Testing

Control Valve Construction  The control valve is roughly divided into 2 parts –Actuator Assembly –Valve Body

Actuator Assembly

Valve Body

Valve types  Globe valve  Ball valve  Butterfly valve

Control Valve Characteristics  Three of the most common characteristics are –Linear –Equal-percentage –Quick-opening  Assumptions: –Valve travel is proportional to controller OP –Pressure difference across the valve is constant –Process fluid is not flashing, cavitating or approaching sonic flow  Selection is based on applications –For quick pressure relief, we may want to use quick-opening –For controlling flow at low rates, we may want to use an equal percentage –For control of flow across a wide-range of flow rates, use linear

Control Valve Characteristics

Failure Characteristics  Not to be confused with Control Valve Characteristics  Also known as “failure position”  “Fail” refers to instrument air or signal failure  When instrument air fails, there is no air to move the control valve  When signal fails, there is no signal to make more or less air go into the control valve

Failure Characteristics  Control valves can fail-open or fail-close  Fail-open means it will open when there is a failure  Open is its default position  It needs air to close –3 psig means open, 15 psig means close  It is also called an air-to-close control valve  What about a fail-close valve?

Failure Characteristics  Control valve failure position selection is generally based on safety considerations: –Is it safer for the valve to go full open or close in the event of failure?  In some control systems, failure position has impact on controller action – be careful! –For such systems, in fail-open valves, increasing OP means closing valve (0% OP  3 psig; 100% OP  15 psig) –In other systems, 0% OP  15 psig; 100% OP  3 psig can be configured

Control Valve Sizing  Control valves are sized according to their Valve Sizing Coefficient, C v  Need to know how to calculate C v  Many equations are available for control valve sizing, but most equations will come up with roughly the same answers  Each control vendor makes a range of control valves with different C v ’s

Excess Capacity  Flow-control loops –Size the control valve for 150 percent of normal flow rate at the normal flow pressure drop or 120 percent of maximum flow rate at the maximum flow pressure drop, whichever results in the larger C v  Level, pressure and temperature-control loops –Size for 180 percent of normal flow at the normal flow pressure drop or 120 percent of maximum flow at the maximum flow pressure drop, whichever results in the larger C v

Sizing Equations - Liquid  Nomenclature –Q (l/s) = Liquid flow rate through valve –Delta P (kPa) = Pressure drop across valve –G (dimensionless) = Specific gravity of liquid at its flowing temperature –SG is defined as density of liquid / density of 4C

Sizing Equations - Steam  Nomenclature –W (kg/s) = mass flow rate –P i (kPa abs) = Inlet pressure –ρ (kg/m 3 ) = Inlet density –C 1 is a valve type-specific factor (See next slide) –K c = Valve capacity correction factor for steam or gas flows when Δ P is less than critical –Δ P (kPa) = Pressure drop across valve –Value in parentheses is in degrees, not radians

Sizing Equations – C 1

Sizing Equations - Gas  Nomenclature –W (kg/s) = mass flow rate –P i (kPa abs) = Inlet pressure –M (g/mol) = Molecular mass –T (K) = Inlet temperature –C 1 is a valve type-specific factor (See next slide) –K c = Valve capacity correction factor for steam or gas flows when Δ P is less than critical –Δ P (kPa) = Pressure drop across valve –Value in parentheses is in degrees, not radians

Example  Size a control valve to control the flow rate of water in a pipe. This control valve is part of a flow-control loop. These are the information given: –Normal flow rate=3000 kg/hr –Normal flow temperature=30ºC –Upstream pressure, Pi=400 kPa abs –Downstream pressure, Po=240 kPa abs –Specific gravity of 30ºC

Control Valve Performance  Control valve is usually the weakest link in a process control loop –Measurement devices can measure with very high accuracy –A 32-bit processor used as a controller can calculate up to many decimal places of accuracy –The control valve, being a mechanical device is subjected to wear and tear

Factors Affecting Performance  Deadband –A region in which the control valve does not respond to controller output  Backlash –Due to looseness in mechanical fittings  Hysteresis –Different behaviours which opening or closing  Process control engineers will only say that the control valve is “sticky”

Deadband  Small OP changes do not affect the PV. Only when the OP has changed more than the deadband will the PV change  Usually due to friction –Air pressure cannot overcome the frictional force resulting in no control valve movement  So more air is introduced into the control valve  When the air pressure finally overcomes the friction, the valve has moved too much

Control Valve Performance Good valve

Backlash  Due to wear and tear  Actuator moves but there is some slack that must be overcome before there is movement  Results in deadtime – results in conservative controller tuning

Hysteresis  The control valve responds to the controller output  But it moves more in one way than the other  Controller gets confused because it has only one set of tuning parameters

Good Control Valve Performance

Terrible Control Valve Performance

Testing Control Valve Performance  Put loop in MAN, do OP step change by small amounts, observe PV  If no change in PV, then continue to change OP in the same direction until you observe PV change  If a PV only responses only after the OP is changed by a total of 1%, the control valve probably has a deadband of 1%  Backlash shows up as deadtime, that is why it is good to use flow loops to determine backlash as flow loops has almost zero deadtime  Hysteresis is easily seen from the process gains you get in the “up” compared to the “down” directions

Assessing Deadband Another way to assess deadband: Tune loop sluggishly; Put loop in AUTO; Make a change in the SP; When you observe a change in the PV, change the SP in the opposite direction; Assess deadband using data from 2 nd step

Plant Testing