5 replies to this topic

[mahfam]

Hi,

I am sizing a Control Valve and Bypass Valve as the attached sketch. During the normal conditions, the Bypass Valve is closed and all the flow passes from the Control Valve with negligible delta P. During the bypass conditions, half of the flow will pass from the Control Valve, and the other half from Bypass Valve. Normally, the delta P of both valves during the bypass conditions should be equal, in order to pass the same flow; however, in my case there is a HEX before Control Valve with 0.7 bar delta P in normal conditions which may decrease during the bypass conditions. In this case, should I assume a smaller delta P for the Control Valve rather than the Bypass Valve (Bypass Valve delta P-Control Valve delta P =HEX delta P during bypass conditions)?

Right now, the maximum opening of Control Valve during bypass conditions is 50%. Can I assume that due to the HEX delta P, if we want to have equal flow in both valves, the Control Valve will open a little to compensate for the delta P?

Thanks,

Attached Files

•  Control & Bypass Valve.jpg   11.91KB   42 downloads

[kkala]

I repeat mahfam's query, with some comments, hopefully being helpful.

I am sizing a Control Valve and Bypass Valve as the attached sketch. During the normal conditions, the Bypass Valve is closed and all the flow passes from the Control Valve with negligible delta P. So this is a line size control valve? Control valves regulating flow (not on/off solenoid valves) are usually of lower size than the line. We assume this valve regulates flow, when bypass valve gets opened.
During the bypass conditions, half of the flow will pass from the Control Valve, and the other half from Bypass Valve. Normally, the delta P of both valves during the bypass conditions should be equal, in order to pass the same flow; ΔP through both valves should be equal, at any case (before HEX installation). Usually bypass valve is designed to have (approximately) same Cv as the control valve, meaning same flow for both valves for about same opening (if main and bypass line are of same diameter, or ΔP along two lines is negligible). Let us assume so for the case.
however, in my case there is a HEX (understood: heat exchanger) before Control Valve with 0.7 bar delta P in normal conditions, which may decrease during the bypass conditions. HEX ΔP during bypass (half flow) would be about 0.7/2^2=0.2 bar.
In this case, should I assume a smaller delta P for the Control Valve rather than the Bypass Valve (Bypass Valve delta P-Control Valve delta P =HEX delta P during bypass conditions)? Yes. To have same ΔP in main and bypass line (under assumptions as above), meaning same flow, control valve ΔP should be lower to ΔP of bypass valve by 0.2 bar.
Right now, the maximum opening of Control Valve during bypass conditions is 50%. Can I assume that due to the HEX delta P, if we want to have equal flow in both valves, the Control Valve will open a little to compensate for the delta P? Yes; it has to be verified through the valve curve (flow vs ΔP). No problem is expected, since reduction of ΔP by 0.2 bar is normally a small change for valve at 50% opening.

[DB Shah]

Right now, the maximum opening of Control Valve during bypass conditions is 50%. Can I assume that due to the HEX delta P, if we want to have equal flow in both valves, the Control Valve will open a little to compensate for the delta P? [font="Arial"]Yes; it has to be verified through the valve curve (flow vs ΔP). No problem is expected, since reduction of ΔP by 0.2 bar is normally a small change for valve at 50% opening.

I differ here,
Case-Normal flow
At normal flow ie 100% flow from Hex + control valve DP across the valve is minimum (Let me assume 0.1 Bar)
So if hex inlet pressure is P1, outlet of CV = P1-0.7-0.1 = (P1-0.8) bar = destination pressure

Bypass case
At bypass flow of 50% flow, pressure drop across hex = (0.5)^2 *0.7 = 0.175 Bar. Destination pressure is fixed = (P1-0.8) hence pressure drop across CV = P1 - (0.175) - (P1-0.8) = 0.625 Bar

Hence we have two operating case-
1- Flow = Q m3/hr & Dp = 0.1 bar
2- Flow = Q/2 m3/hr & DP = 0.625 bar --- CV opening is ~ 50% as given.
If I assume equal percent characteristics for the CV the opening for normal case (case-1) will be about 90%. Hence there is a wide difference of opening between normal case and bypass case.

We have such loops to control downstream temperature (reactor). In such cases the selection of characteristics of the valve is very critical (Equal percent, quick opening, split range).

[mahfam]

DB Shah:

Thanks. Actually, in my case, the downstream pressure is not very critical. This bypass valve is only used in order to bypass the heat exchanger. So, if the destination pressure is not the same in both cases, that's not a big deal.

[mahfam]

Thanks a lot.

[S.AHMAD]

Mahfam
1. Why you want to regulate the flow?
2. In most cases, bypassing heat exch. is to control the outlet temperature by regulating flowrate and three-way valve is normally used for this purpose.

Kumar
1. May I ask you the basic question, what is the purpose of bypass?
2. If you know the purpose then you will be able to determine the size or whether bypass is required or not.