Valves Flashcards
What is the installed characteristic?
The installed characteristic is a plot of the delivery flowrate against the stem position
What does the installed characteristic depend on?
It depends on the ratio of the pressure drop through the valve to the total pressure drop.
Which valve trim should be chosen?
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
What is the turndown ratio of a valve?
The minimum design flow/ minimum controllable flow
What is the rangeability of the valve?
maximum controllable flow/ minimum controllable flow
How are turndown and rangeability related?
The turndown is usually 70% of the rangeability
How does the size of the valve relate to the size of the pipeline
The valve is always smaller than the pipeline in which it is installed
The equation for linear trim in terms of Cv
Cv = Cv max * x
The equation for linear trim in terms of Q
Q = Q max* x
When does Q max occur?
When the stem position, x = 1.
The valve is fully open
What is delta hv?
hv = height of pump characteristic - height of system curve
Describe a valve with a linear trim?
A change in the stem position will cause an equal change in the flowrate through the valve.
How does Cv, max relate to the normal Cv?
Cv, max should be 40 - 70 % greater than the normal Cv
What are the 2 most common valve trims?
Linear and Equal % trim
Describe the procedure for determining if a trim is suitable?
- Plot the system curve on top of the characteristic curve.
- Calculate delta hv for the range of flowrates
- Calculate Cv using Cv = Q/sqrt(rhow* g *hv)
- Calculate x using Cv/ Cv, max
What is the equation for the stem position related to Cv and Cv, max?
x = Cv/ Cv,max
What is the equation for Q?
Q = Cv * sqrt (rhowgdelta hv)
rho is the density of water because water is the reference fluid with which the valves are tested.
How does the inherent characteristic from the installed characteristic?
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.
What are the 2 methods for controlling flow from a pump?
- Altering the pump speed.
2. Throttling the output from a pump using a control valve.
How could the pump speed be altered for ac and dc pumps?
Gears for an ac pump - discrete number of speeds and added maintenance
Dc pump speed control - the problem is cooling a large dc pump
How does throttling output change the pump speed?
Changes the resistance to the flow and the gradient of the system curve. Energy is wasted across the valve.
What are the general types of control valve?
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.
What does the final control element consist of?
An actuator - converts the signal to a force
Device which will respond to the actuator force
What could an actuator do in a valve?
It could change the stem position, could change the velocity of the rotating device, change the power delivered to an electrical load.
