3 replies to this topic

[Vibha_101]

How to decide pressure drop across control valve for liquid & gas service?

[breizh]

hi,

Consider the docs attached . You should also give a try with the search engine in this Forum .

I do remember a document issued by Chris Haslego  ( the owner of this forum) .

Good luck

Breizh

 

 

Edited by breizh, 31 August 2020 - 04:36 AM.

[Chemitofreak]

How to decide pressure drop across control valve for liquid & gas service?

 

1) One needs to carry out hydraulics. From the source equipment until the upstream of the control valve (source pressure - line pressure drop). Similarly from the destination up to the downstream of the control valve (destination pressure + line pressure drop). This difference is the drop in the control valve

 

2) If the control valve is on the pump or compressor discharge you can use 0.7 bar (liquuid) and 0.2 to 0.3 (gases) respectively. This is a good engineering practice because of operability of the valve and energy consumption.

Edited by Chemitofreak, 31 August 2020 - 05:14 AM.

[Subhendu]

Control valve pressure drop (for non-pumped circuits) can be calculated as below:

1. Select source and destination point in the circuit, where in the control valve is located. Ideally these points should be fixed pressure points, i.e. the pressure at these points should not change with change in flow. For example, a vessel/column with pressure control can be considered as fixed pressure point. 
2. Estimate the upstream control valve pressure by Source pressure + static head (if any) - frictional loss through equipment and instruments - line loss (calculate the line loss for one case, say maximum flow case, for equipment pressure drop consider allowable pressure drop per equipment datasheet, For other flow cases pro-rate the pressure drop values as delta P₂ = delta P₁ x (M₂/M₁)², where delta P₁ = Maximum flow pressure drop, delta P₂ = other flow case pressure drop, M's are the corresponding mass flow rate). Thus you will have three set of values for control valve upstream pressure corresponding to Maximum , minimum and normal flow cases.
3. Estimate the control valve downstream pressure by destination pressure + line loss + frictional loss through equipment and instruments + static loss. In similar way like point 2, arrive at 3 values corresponding to Maximum , minimum and normal flow cases,
4. Difference between control valve upstream and downstream pressure will give you the control valve pressure drop for the corresponding case. Please be advised, for liquid service minimum control valve pressure drop is 0.7 bar or 10 psi for stable operation and for vapor case it can be considered 0.3 bar.

For pumped circuit or compressor circuit it is recommended to take the control valve pressure drop from the corresponding pump/compressor calculation.

For pumped circuit, following guidelines can be followed

Control valve pressure drop shall be maximum among

1. minimum 0.7 bar for rated flow case
2. minimum 1 bar for normal flow case
3. 25% of system frictional loss for rated flow case
4. 33% of system frictional loss for normal flow case.