INFORMATION ABOUT CONTROL VALVES

      Que. :  What is a control valves ?

Ans. :  A control valve is the final control element, which directly changes the valve of the manipulated variable by changing the rate of flow of control agent.

A control valve consists of an operator and valve body. The operator provides the power to vary the position of the valve plug inside the body. The plug is connected to the operator by a stem, which slides through a stuffing box. The air signal from the controller is applied above the diaphragm. The increasing air signal from the controller is applied above the diaphragm. An increasing air signal will push the operator stem downwards against the force exerted by the spring on the diaphragm plate. The valve is adjusted in such a way that the plug starts moving when 3 psi is applied to the diaphragm and touches the seat when 15 psi is applied gm. Thus an increase in air pressure will close the valve. Hence the home "Air to Close". Another type is "Air to open", such that 3 psi on the diaphragm the value is closed and 15 psi air signal it in fully open.

Que. :  What are the different types of control valves ?

Ans. :  The commonly used control valves can be divided as follows.

1. Depending on Action.

2. Depending on the Body.

1. DEPENDING ON ACTION :

Depending on action there are two types of control valves, (1) Air to close, (2) Air to open.

2. DEPENDING ON BODY :

1. Globe valves single or double seated.

2. Angle valves.

3. Butterfly valves.

4. Three way valves.

Que. :  What is the use of single seated valve ?     

Ans. :  The single seated valve is used on smaller sizes, and in valve of larger sizes, where an absolute shut off is required. The use of single seated valve is limited by pressure drop across the valve in the closed or almost closed position.

Que. :  What is the use of double seated valve ?

Ans. :  In double seated valves the upward and downward forces on the plug due to reduction of fluid pressure are nearly equalized. It is generally used on bigger size valves and high pressure systems. Actuator forces required are less i.e. A small size actuator.

Que. :  What is CV of a valve ?

Ans. :  CV is the capacity of a valve and is defined as :

"No of gallons per minute of water which passes through a fully open valve at a pressure drop of 1 psi.

The valve coefficient CV is proportional to the area 'A' between the plug and valve seat measured perpendicularly to the direction of flow.

Que. :  What are the different types of actuators ?

Ans. :  The different types of actuators are :

1. Diaphragm Operated.

2. Piston Operated.

Que. :  What types of bonnets would you use of high temp. and low temp. ?

Ans. : 

HIGH TEMP. : Bonnets are provided with radiation fins to prevent glad packing from getting damaged.

ON VCF LOW TEMP. : Extended bonnets are used to prevent gland packing from getting frcored.

Que. :  How will you work on a control valve while it is line ?

Ans. :  While the control valve is in line the control valve has to be got by passed and secondly the line has to be drained and depressurized.

Que. :  What is the use of  a valve positioner ?

Ans. :  The valve positioner is used for following reasons :

1. Quick Action control valve.

2. Valve hysteresis.

3. Valves used on viscous liquids.

4. Split Range.

5. Line pressure changes on valve.

6. Valve Bench set not standard.

7. Reversing valve operation.

Que. :  When can a by pass be not used on a positioner ?

Ans. :  A by pass on a positioner cannot be used when :

1. Split Range operation.

2. Reverse Acting Positioner.

3. Valve bench set not standard.

Que. :  What is the use of link connected to the valve positioner ?

Ans. :  The link serves as the feed back to the value. Ant valve movement is sensed by this link. Sometimes due to line pressure changes on H.P. service the valve position may be changed, the link in turn senses this change and the positioner will produce an output which will operate the valve to the original position.

Que. :  What is the use of  booster relays ?

Ans. :  Booster relays are essentially air load, self contained pressure regulators. They are 

classified into three broad groups :

1. Volume Boosters : These are used to multiply the available volume of air signal.

2. Ratio Relays : Use to multiply or divide the pressure of an input signal.

3. Reversing Relays : This produces a decreasing output signal for an increasing input signal.

Que. :  What is the use of Angle valves ?

Ans. :  Angle valves are used where very high pressure drops are required and under very severe conditions. Where the conventional type of valve would be damaged by erosion.

Que. :  What is the use of butterfly valves ?

Ans. :  Butterfly valves are used only in systems where a small pressure drop across the valve is allowed. The butterfly is fully open when the disc rotates by 90. A drawback of this valve is that even a very small angular displacement produces a big change in flow.

Que. :  What is the use of three way valves ?

Ans. :  Three way control valves are only used on special systems, where a dividing or mixture of flows according to a controlled ratio is required.

Que. :  What is a cage valve ?

Ans. :  A cage valve uses a piston with piston ring seal attached to the single seated valve "plug". Here the hydrostatic forces acting on the top or the piston or below the valve plug tend to cancel out. The seat ring is clamped in by a cage. Cage valves are generally used for noise reduction.

Que. :  What are the advantages of Camflex valves ?

Ans. :  Camflex valves are intermediates between globe valve and butterfly valve. The plug rotates 60' for full opening.

ADVANTAGE :

1. Actuator forces required are very less.

2. Extended bonnet and hence can be used on any service i.e. on high temp. and very low temp.

3. Variations in flow.

4. Light weight.

Que. :  What are the different types of plugs ?

Ans. :  The different types of plugs are generally used are :

( 1 ) V. port plug          ( 2 ) Contoured plug

V. PORT PLUG :

Ported plug are generally used on double seated valves. This is because ported plugs, have a more constant off balance area.

CONTOURED PLUGE :

Contoured plugs are generally used on single seated valve with small trim sizes.

Que. :  What are the different valve characteristic ?

Ans. :  The different types of valve characteristic are :

1. Linear                     2. Equal Percentage               3. Quick Opening.

LINEAR : The valve opening to flow rate is a linear curve

EQUAL PERCENTAGE :

For equal increments of valve opening it will give equal increment in flow rate range. At small opening the flow will also be small.

QUICK OPENING :

At small opening the increments in flow rate is more. At higher opening the flow rate becomes steady.

QUE. : What is a solenoid valve ? Where it is used ?

Ans. :  A solenoid is electrically operated valve. It consist of a solenoid ( coil ) in which a magnetic plunger moves which is connected to the plug and tends to open or close the value. There are two types of solenoid valves :

1. Normally open                    2. Normally closed

USE : It is used for safety purpose.

Que. :  How will you change the valve characteristics with positioner ?

Ans. :  The positioner contains different types of came in it. selection of the proper can the valve opening characteristics can be changed.

Que. :  How will you change the action of a control valve ?

Ans. : 

1. If the control valve is without bottom cap. The actual needs to be changed.

2. If bottom cap is provided.

a) Disconnect the stem from  the actuator stem.

b) Separate the body from the bonnet.

c) Remove the bottom cap and the plug from body.

d) Detach the plug from the stem by removing the pin.

e) Fix the stem at the other end of the plug and fix the pin back.

f) Turn the body upside down. Connect it to the bonnet after inserting the plug and stem.

g) Connect back the stem to the actuator stem.

h) Fix back the bottom cap.

i) Calibrate the valve.

 Que. :  How will you select the control valve characteristics ?

Ans. :  The graphic display of flow various lift shows then the Desired or inherent characteristic is changed by variations pressure drop. This occurs as the process changes from condition where most of pressure drop takes place at the control valve is a condition where most of the pressure drop is generally distributed through rest of the system.

% Flow : This variation in where most of the total drop take place is one of the most important aspects is choosing the proper valve characteristics for give process.

Flow control : Normally Equal percentage valve is used.

Pressure Control : Normally linear valve is used to maintain a constant pressure drop.

Temp. Control : Normally equal percentage valve is used.

Liquid Level Control : Normally linear valve is used.

Basically in selecting a valve characteristic two important point have to be taken into account.

a) There should be a linear relationship between the position of the plug and the flow through the valve in a wide range of change in the pressure drop across the valve.

b) The pressure drop across a valve should be as low as possible.

Que. :  An operator tells you that a control valve in a stuck ? How will you start checking ?

Ans. : 

1. First of all get the control valve is passed from operation.

2. Check the lingual to the diaphragm of the control valve.

3. Disconnect it possible the actuator stem from the control valve stem.

4. Stroke the actuator and see whether the actuator operates or not. It not then the diaphragm may be punctured.

5. If the actuator operates connect it back to the plug stem stroke the control valve. If it does not operate loosen the gland nuts a bit and see if it operates. If it does not then the control valve has to be removed from the line to w/shop.

Que. :  Where is an Air to close and Air to open control valves used  ?

Ans. :

AIR TO CLOSE :

1. Reflux lines.

2. Cooling water lines.

3. Safety Relief services.

AIR TO OPEN :

1. Feed lines.

2. Steam Service.

Que. :  Why does control valve operate at IS psi ?

Ans. :  On higher pressure the actuator sizes becomes bigger in area. The actual force produced by the actuator.

Force = Pressure  x  Area.

           = 15 psi  x Area If Area = 15"

Force produced = 15 psi  x  25" pounes.