[From：来自网络] [Author：admin] [Date：11-11-15] [Hit：]
It’s interesting to see how big the pressure drop through the reduced bore ball valves. It shall be examine case by case but below calculation could give us a general overview regarding why the default choice of ball valves will be reduced bore. Let’s set up a general rule first. If the valve is serving a piggable line then it shall be full bore ball valves. If it serves a blow down application it should be full bore. If it serves a general process application then the only consideration is a pressure drop. If the pressure drop isn’t significant by using reduced bore, then reduced bore ball valves should be use instead of full bore in term of cost and weight efficiency for offshore applications. Our case study will be for liquid applications.
Pressure drop formula
We can get pressure drop formula by derived it from liquid flow equation at a valves:
If QL = CV * (Delta P/GL)^0.5 then,
Delta P = GL * (QL / CV)^2
Where,
QL = Liquid Flow Rate (US gpm)
GL = Liquid Specific Gravity
CV = Coefficient of Flow
Delta P = Pressure Drop (psi)
Example,
Given data:
QL = 1800 bbl/day = 52.5 USgpm
GL = 0.995
Line Size = 4 inch
Pressure Rating = ANSI 600
Question: What we should use for this application, full bore or reduced bore shutdown valves?
Answer:
From a vendor catalogue it is mentioned that the Cv for 4 inch ball valve is as below:
4” Full Bore Ball Valve CV = 1850
4 x 3 inch Reduced Bore Ball Valve CV = 600
Pressure drop calculation for 4” full bore ball valves:
Delta P = 0.995 * (52.5 / 1850)^2 = 0.0008013 psi.
Pressure drop calculation for 4 x 3 inch reduced bore ball valves:
Delta P = 0.995 * (52.5 / 600)^2 = 0.0076 psi.
From this calculation example, we can conclude that by using a reduced bore ball valve is acceptable since it isn’t affecting the system pressure drop heavily. Also since this application isn’t for blow down and put at piggable line then we can conclude that reduced bore is best choices for this particular application.