CONTROLVALVES
1 What is a controlvalves?
A control valve is the final controlelement, which directlychanges the valve of the manipulated variable by changing the rate of flow of controlagent.
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 operatorby a stan, which slides through a stuffing box. The air signal from the controller is applied above the diaphragm. The increasing air signalfrom the controller is applied abovethe diaphragm. An increasing air signal will push the operator steam downwards against the face exentedby the spring on the diaphragm plate. The valve is adjustedin such a way that the plug starts moving when 3 psi is applied to the diaphragmand touches th% reatwhen 15 psa is a-pd) D ,"L D gm. Thus an increasein air pressure will close the valve. Hence the home "Air to Close". Another type is "Airto open", such that 3 psi on the diaphragmthe value is closed and 15 psi air signal it in fully open.
2 What are the different types of control valves?
The commonlyused control valvescan be devided as follows.
l. 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 singleor double seated.
2. Angle valves.
3. Butterflyvalves.
4. Three way valves.
3 What is the use of single seatedvalve?
The single seated valve is usedon smaller sizes, and in valve of larger sizes, where an absolute shut off is required. The use of single seated valveis limited by pressure drop acrossthe valve in the closed or almost closed position.
4 What is the use of double seated valve?
In double seated valves the upward and downward forceson the plug due to reduction of fluid pressure are nearly equalized. It is generallyused on bigger size valves and high pressuresystems. Actuator forces required are less i.e. A small size actuator.
5 What is Cv of a valve?
Cv is the capacityof a valve and is defind as :
"No of gallons per minute of water which passes througha fully open valve at a pressure drop of 1 p.s. The valve codfficient Cv is proportional to the area 'A' between the plug and valve seat measured
perpendicularly to the direction of flow.
6 What are the different types of actuators?
The differenttypes of actuatorsare :
1. Diaphragm Operated. 2. PistessOperated.
7 Whattypes ofbonnets wouldyou use ofhigh temp.and low temp. ?
HIGH TEMP. : Bonnetsare provided with adiation fins to preventglad packing from getting damaged. ON VCF LOW TEMP. : Extendedbonnets are used to prevent gland packingfrom getting frcored.
8 How will you work on a controlvalve while it is line?
While the controlvalve is in line the control valve has to be got by passed and secondlythe line has to be drainedand depressurized.
9 What is the use of a valvepositioner?
The valve positioner is used for following reasons:
l. 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.
10 Whencan a by pass be not used on a positioner?
A by pass on a positioner cannot be used when:
l. Split Range operation.
2. ReverseActing Positioner.
3. Valve bench set not standard.
11 What is the use of link connected to the valvepositioner?
The link servesas the feed back to the value.Ant valve movementis sensed by this link. Sometimes due toline pressure changeson H.P. service the valve position may be changed,the link in turn sensesthis change and the positioner will produce an output which will operate the valve to the original position.
12 What is the use of booster relays?
Booster relays are essentially air load, self contained pressure regulaters. They are classified into three broad groups :
l. Volume Boosters: These are used to multiply the available volumeof air signal.
2. Ratio Relays: Use to multiply or divide the pressure of an input signal.
3. ReversingRelays: This produces a decreasing outputsignal for an increasing input signal.
13 What is the use of Angle valves?
Angle valves are used where very high pressure drops are required and undervery severe conditions. Where the conventional type of valve would be damaged by erotion.
14 What is th e use of butterfly valves?
Butterfly valves are usedonly in systems where a small pressuredrop across the valve is allowed. The butterfly is fully open when the disc rotatesby 90. A drawback of this valveis that even a very small angular displacement producesa big change in flow.
15 What is the use of three way valves?
Three way control valves are only used on special systems, where a dividing or mixlug of flows according to a controlled ratio is required.
16 What is a cage valve?
A cage valve uses a piston with piston ring seal attached to the single seatedvalve "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.
17 Whatare the advantages of comflex valves?
Comflex valvesare intermediates between globevalve and butterfly valve. The plug rotates 60' for full opemng.
ADVANTAGE:
l. Actuater forces required are very less.
2. Extendedbonnet and hence can be used on any service i.e. on high temp. and very low temp.
3. Vanationsin flow.
4. Light weight.
18 Whatare the different typesof plugs ?
The differenttypes of plugsare generally used are : (1) V. port plug (2) Contoured plug
VPORTPLUG:
Ported plug are generally used on double seated valves. This is because ported plugs, have a more constant off balance area.
CONTOUREDPLUGE :
Contoured plugs are generallyused on single seated valve with small trim sizes.
19 What are the different valve characteristic?
The differenttypes of valve characteristic are:
l. Linear 2. Equal Percentage 3. Quick Opening. LINEAR : The valve opening to flow rate is a linear curve
EQUAL PERCENTAGE: Forequal increments of valveopening it will give equal increment in flow raterange. At small opening the flow will also be small.
QUICK OPENING: At small openingthe increments in flow rate is more. At higher openingthe flow rate becomes steady.
20 Whatis a solenoid valve? Where it is used?
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 openor close the value. There are two types of solenoid valves:
l. Normally open 2. Normally closed
USE : It is used for safety purpose.
21 How will you change the valve characteristics with positioner?
The positioner contains different types of came in it. selection of the proper can the valve opemng characteristics can be changed.
22 How will you change the action of a control valve?
l. If the control valve is withoutbottom cap. The actual needs to be changed.
2. Ifbottom cap is provided.
a) Disconnectthe stem from the actuater stem.
b)Separate the body from the bormet.
c) Remove the bottomcap and the plug from body.
d) Datach the plug from the stem by removingthe pin.
e) Fix the stem at the other end of the plug and fix the pin back.
f) Turn the body upside down. Cormectit to the bonnet after inserting the plug and stem. g) Cormect back the stem to the actuator stem.
h) Fix back the bottom cap.
i) Calibrate the valve.
23 How will you select the control valve characteristics?
The graphic displayof flow verious lift shows then the deside or inherent characteristic is changed by variations pressure drop. This occurs as the processchanges from condition where most of pressure drop takes place at the control valve is a condition wheremost of the pressure drop is generally distributed through rest of the system.
% Flow: This variationin where most of the total drop take place is one of the most importantaspects 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 pressuredrop.
Temp. Control: Normally equal percentage valve is used.
Liquid Level Control: Normally linear valve is used.
Basically in selecting a valvecharacteristic two importantpoint have to be taken into account.
a) There should be a linear relationship between the position of the plug and and the flow throughthe valve in a wide range of change in the pressure drop across the valve.
b) The pressure drop accross a valveshould be as low as possible.
24 An operator tells you that a control valve in a steech ? How wiil you stent checking?
l. First of all get the control valve is passed from operation.
2. Check the lingualto 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 operatescormect 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 controlvalve has to be removed from the line to w/shop.
25 Whereis an Air to close and Air to open control valvesused ?
AIR TO CLOSE:
l. Reflux lines.
2. Cooling water lines.
3. Safety Relief services. AIR TO OPEN:
l. Feed lines.
2. Steam Service.
26 Why does control valve operate at IS psi?
On higher pressurethe actuator sizes becames biggerin area. The actual force produced by the actuator.
Force
=Pressure x Area.
= 15 psi x Area If Area = IS"
Force produced= 15 psi x 25" pounes.
