Valves

Question 1

What is the installed characteristic?

Answer 1

The installed characteristic is a plot of the delivery flowrate against the stem position

Question 2

What does the installed characteristic depend on?

Answer 2

It depends on the ratio of the pressure drop through the valve to the total pressure drop.

Question 3

Which valve trim should be chosen?

Answer 3

The valve trim that gives the most linear relationship on the installed characteristic or the valve trim that gives the largest rangeability across a range of x

Question 4

What is the turndown ratio of a valve?

Answer 4

The minimum design flow/ minimum controllable flow

Question 5

What is the rangeability of the valve?

Answer 5

maximum controllable flow/ minimum controllable flow

Question 6

How are turndown and rangeability related?

Answer 6

The turndown is usually 70% of the rangeability

Question 7

How does the size of the valve relate to the size of the pipeline

Answer 7

The valve is always smaller than the pipeline in which it is installed

Question 8

The equation for linear trim in terms of Cv

Answer 8

Cv = Cv max * x

Question 9

The equation for linear trim in terms of Q

Answer 9

Q = Q max* x

Question 10

When does Q max occur?

Answer 10

When the stem position, x = 1.

The valve is fully open

Question 11

What is delta hv?

Answer 11

hv = height of pump characteristic - height of system curve

Question 12

Describe a valve with a linear trim?

Answer 12

A change in the stem position will cause an equal change in the flowrate through the valve.

Question 13

How does Cv, max relate to the normal Cv?

Answer 13

Cv, max should be 40 - 70 % greater than the normal Cv

Question 14

What are the 2 most common valve trims?

Answer 14

Linear and Equal % trim

Question 15

Describe the procedure for determining if a trim is suitable?

Answer 15

1. Plot the system curve on top of the characteristic curve.
2. Calculate delta hv for the range of flowrates
3. Calculate Cv using Cv = Q/sqrt(rhow* g *hv)
4. Calculate x using Cv/ Cv, max

Question 16

What is the equation for the stem position related to Cv and Cv, max?

Answer 16

x = Cv/ Cv,max

Question 17

What is the equation for Q?

Answer 17

Q = Cv * sqrt (rhowgdelta hv)

rho is the density of water because water is the reference fluid with which the valves are tested.

Question 18

How does the inherent characteristic from the installed characteristic?

Answer 18

The inherent characteristic is the theoretical performance of the valve at constant pressure drop. The installed characteristic takes into account that a pressure drop will arise across the valve due to flow.

Question 19

What are the 2 methods for controlling flow from a pump?

Answer 19

1. Altering the pump speed.

2. Throttling the output from a pump using a control valve.

Question 20

How could the pump speed be altered for ac and dc pumps?

Answer 20

Gears for an ac pump - discrete number of speeds and added maintenance
Dc pump speed control - the problem is cooling a large dc pump

Question 21

How does throttling output change the pump speed?

Answer 21

Changes the resistance to the flow and the gradient of the system curve. Energy is wasted across the valve.

Question 22

What are the general types of control valve?

Answer 22

Globe valve = movable disk element and stationary ring seat. These are used in regulating flowrate instead of having “all or nothing”.
Diaphragm valve.
Non-return valve = allows a medium to flow in only one direction
Ball valve = there is an opening in the middle called a port.

Question 23

What does the final control element consist of?

Answer 23

An actuator - converts the signal to a force

Device which will respond to the actuator force

Question 24

What could an actuator do in a valve?

Answer 24

It could change the stem position, could change the velocity of the rotating device, change the power delivered to an electrical load.