HOMEWORK ASSIGNMENTS:
Control valve sizing exercises:
Q1
A1
Q2
A2
Why is every control valve a FCV?
The control valve directly controls the flow of fluid by varying passage size. By controlling flow
rate, other process quantities such as pressure, temperature, and liquid level of process fluid
are also controlled.
For instance, at normal condition of liquid-vapor separator, the LCV is 40% open. When the
inlet flowrate increases, the liquid level also increases. To maintain the liquid level, the
opening of LCV will increase to alter the flow of the liquid.
Mr. Koteswararao is not sure how to find out Cv of a valve. Have a brainstorming session in
your class and try to find the answer.
In order to hold CV performance test, throttling valve and flow sensor are placed on upstream
of the valve. Pressure trap and throttling valve are also placed on the downstream. The
illustration is shown below.
Then, the valve is tested at a minimum of three different flow/pressure combination for
each valve travel position. When the data are recorded, Cv can be calculated using the
formula below.
If the test setup is correct, the CV calculated at each flow condition should broadly be the
same. Any deviation from the average of the three points may indicate the presence of
cavitation and should be investigated
Q3
Mr. Rahul is trying to select appropriate LCV size for toluene at 20 oC, 55 m3/h (normal flow),
5 barg upstream conditions and with 1 bar pressure drop across the valve. Can you help him?
A3
Control valve size recommendation: 75mm (3 in) with line sizing 100 mm ( 4 in)
Attachment:
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Q4
A4
What is cavitation in control valve?
the liquid pressure drops to value below its vapor pressure, causing a liquid to vaporize into
vapor bubbles.
Q5
A5
How do you fix cavitation problem?
increase the downstream pressure by installing a flow restrictor if possible or reducing the
pipe size of a short piece downstream
Q6
A6
How do you mean by valve CV is 25?
25 gallons per minute (gpm) water at 60°F that will pass through a full open valve with a
pressure drop of 1 psi.
Q7
A7
What are different trim characteristics in control valves? What to use which?
1) Linear
- Used in liquid level or flow loops
- Used in systems where the pressure drop across the valve is expected to remain
fairly constant
2) Equal percentage
- Used in processes where large changes in pressure drop are expected
- Used in temperature and pressure control loops
3) Quick opening
- Used for frequent on-off service
- Used for process where “instantly” large flow is needed (i.e safety systems or
cooling water system)
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Q8
A8
What standard needs to be used for control valve sizing?
ISA 75.01.01
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