Control Valve – Theory & SizingControl Valve – Theory & Sizing

The control valve, commonly named the final control element of control contains a pneumatic device that converts the control signal from the controller in action, regulating the process fluid flow.

Valves make up approximately 15% of the total expenditure for the materials and equipments for the refinery or chemical processing industry.

Valves are used to handle variety of liquids , gases , steam, air etc.

Specification Of Valves Specification Of Valves

Flow medium Flow : kg/hr or Nm3/hr Max/Min/Normal Inlet pressure : kg/cm2 Max/Min/Normal Outlet pressure:kg/cm2 Max/Min/Normal Max. allowable diff. Pressure : kg/cm2 Density of medium : kg/m3 Viscosity Cv : Valve Flow Coefficient

Operating temperature - Deg. C Design Pressure / temp. End connection MOC desired leak Tightness required Actuator : Pneumatic / electrical Max. compressed air available : kg/cm2 Fail Safe Action

Specification Of ValvesSpecification Of Valves

Category Of ValvesCategory Of Valves

ON - OFF SERVICES :- Gate, Ball, Diaphragm, Plug, Butterfly valves.

THROTTLING SERVICES :- Globe, Butterfly, Diaphragm, Pinch valves.

NON - REVERSE FLOW :- Check valves.

Gate ValvesGate Valves

The Gate Valves are characterized by a sliding gate which is moved by the stem perpendicular to the direction of flow.

Application :- It is used for on-off application. Suited for high temperature and pressure use with variety of fluids. They are not primarily used for slurries, viscous fluid etc.

ADVANTAGES : Low pressure drop when

fully open and tight sealing

DISADVANTAGE Causes Vibration, seat disc wear in partial open condition. Slow response characteristic and require large actuating force.

Gate ValvesGate Valves

Globe Valves

In Globe Valves disc or plug is moved on and off the seat.The seat opening is directly proportional to the travel of the plug.It is short stem travel, high seating capacity, large pressure drop & high flow controllability. Application :- It is used primarily for throttling purposes. It may be considered a general purpose flow control valve. Specifically very widely used for high temp. application.

Globe Valves

ADVANTAGES : Faster to open or close, most reliable form of seating, throttling to control the flow to any desired

degree, positive shut-off

DISADVANTAGE Seat disc wear in partial open condition.

Plug Valves

It consist of body, plug and cover. The plug is tapered or cylindrical. In the open position, the bore in the plug connects the inlet and outlet ends of the valve providing straight line flow.

Application :- The plug valves are extensively used in refinery, petrochem and chemical industries and for general purpose involving on off services.

ADVANTAGES : Normally small in size, require less headroom and available in wide range of materials. They provide tight shut off, quick opening and low pressure drop.

DISADVANTAGE Plug valve may be subjected to galling.

Plug Valves

Ball Valves

It is improvisation of the plug valve. It is basically a ported sphere in a housing.The seat matching the ball is circulars so that the seating stress is circumferentially uniform. The seats are usually made up of PTFE which is inert to all chemicals, has low coefficient of friction and resiliency.

Application :- It is used for flow & pressure control and shut off and for corrosive fluids, slurries, normal liquid & gases. For high temp. & pressure

Ball Valves

ADVANTAGES : Low pressure drop, tight shut off, quarter turn operation, easy to maintain, low torque. They are small in size and low in weight.

DISADVANTAGE PTFE seats are subjected to extrusion if the valve is used for throttling. Fluid trapped in the ball in the closed position may cause problem of build up of vapor pressure & corrosion.

Butterfly Valves

They are rotary valves with a disc rotating at right angles within a pipe section body. The stem passes through this disc and is supported by both ends of the body.The shaft is ensured to the disc either by bolts or pins. Rotating the stem through 90 degrees. Fully opens or closes the valves. Application :-Large flow control, Low pressure application where leakage is unimportant.

ADVANTAGES : Simple , compact form, quick opening, good control,large flow, low pressure drop and low weights and costs.

DISADVANTAGES: No tight shut off.

Butterfly Valves

Diaphragm Valves

The flexible diaphragm is connected to a support member known as compressor, which represent the closure member. When the valve is opened the diaphragm is lifted out of the flow path and the fluid has a smooth streamlined flow. When the valve is closed the diaphragm is tightly seated against the seat in the body known as the weir. The diaphragm is of NEOPRENE Application :- Used for corrosive liquids at low temp. and pressure .

ADVANTAGES : The diaphragm completely keeps the working parts in isolation from the process fluids. Simple in construction, easier to operate and maintain.

DISADVANTAGES: Not for high temp. and pressure.

Diaphragm Valves

Control Valve – Principal Of Control Valve – Principal Of Operation Operation

It is based upon balanced forces between pneumatic force from the diaphragm against a mechanical force produced by the actuator spring.The control valve works with a pneumatic signal 3 to 15 psi. The motion starts at 3 psi and ends at 15 psi.

Control Valve Operational FormsControl Valve Operational Forms

AIR TO CLOSE:- 3 PSI – FULL OPEN 15 PSI – FULL CLOSE

AIR TO OPEN :– 3 PSI – FULL CLOSE 15PSI – FULL OPEN At pressure higher than3 psi applied on diaphragm,it will start

overcoming the spring force and move diaphragm plate which in turn moves stem connected and operates the plug.

Fail Safe OperationFail Safe Operation

AIR FAIL TO OPEN:- AIR FAIL TO CLOSE:-

CONTROL VALVE- HART POSITIONER

ACTUATOR

VALVE BODY

VENT PORT

POSITIONER

YOKE

Actuator Body Trim Diaphragm Diaphragm plate Actuator stem Actuator spring Seat

Components Of Control ValveComponents Of Control Valve

Travel Indicator Valve stem Gaskets Yoke Hand wheel

Component Of Control ValveComponent Of Control Valve

DOMESPRING

DIAPHRAGM

AIR SIGNAL

STEMCOUPLER

INDICATORSCALE

GLAND

PLUG

VALVE SEAT

INLET

OUTLET

VALVE BODY

BONET

MOTOR OPERATED VALVE

TerminologyTerminology

Flow Co-efficient (Cv) :- The flow of water (G=1, T= 6 to 34 deg. C) through the valve at full lift in U.S gallon per minute with a pressure drop across the valve of 1 psi.

VALVE LIFT %

FLO

W %

EQUAL %

LINEAR

QUICK OPENING

Characteristic Characteristic CurvesCurves

Linear :-Flow is directly proportional to lift at constant pressure drop

Equal Percentage :- Flow changes by a constant percentage of

its instantaneous values for each unit change in lift.

Modified flow:- Combination of linear and equal percentage

Quick Opening :- Flow increases rapidly in a linear relationship with plug lift reaching a max. value at a low lift.On-Off service

Guidelines For Control Valve Guidelines For Control Valve SpecificationSpecification

Valve trim is of stainless steel Harden SS or Stellite is used for Pressure drop of 150 psi or more Steam Slurry fluids More temp Teflon packing for les than 200 deg. Temp Asbestos used above 200 deg temp

Valve PositionerValve Positioner

Valve requiring extra force due to process condition such as Large pressure drop,Temp control,Large size , Split range control