Tandem valves design

[Bryantan]

My situation is to design tandem butterfly valves to control the exchanger outlet temp. B valve is in exchanger bypass, A valve is located before exchanger. These two valves were machanically linked, then when A valve opens at X degree, B valve opens at 90-X degree. As a rule, usually, the bypass line is one size less than the main, so as the B valve. And also bypass is designed to make 10% of the total flow goes through it in normal operation.
Since the exchanger pressure drop is pretty high, calculation shows B valve openning is less than 10 degree, A valve opens more than 80 degree. Also B valve instead A valve will control the loop as A valve has only very little pressure drop.
Then 2 questions as below:
First, if the B valve is working at less than 10 degree openning, any problem for the control stability? Or do I need to decrease B valve size to make it open at more than 10 or 15 degree?
Second, Since A valve does not contribute the control a lot even when 50% flow goes through bypass, is there any possibility to delete it? and any negative effects on the control accuracy and flexibility?

 

[sshep]

Hello Bryantan,
You state above that the bypass is designed to make 10% of the total flow go through it at normal operation. It is more correct to say that the exchanger is designed to meet its design temp with 10% of the flow going through the bypass, because the existing layout of the bypass has nothing to do with the amount of flow needed in the bypass.

It will be best for you to define a table for your operating space showing the amount of flow and dP through A & B to meet the temp objectives for various process conditions. This analysis should include consideration of the exchanger performance as a result of process condition changes on the other side of the exchanger. Only with this type analysis you can see if A is required, or if a hand globe valve (or orifice plate) can bring the existing B valve to a more open condition.

With respect to stability, there are a number of disturbances which can affect temperature stability in addition to just the flow through B, including variation in total flow (including by dP changes), inlet temp, and flow/temp on the otherside of the exchanger- these drive the changes in B position, and need to be considered. Although having B open more is probably desirable in general, your situation is not so bad as it apears- i.e. it is not like B is less than 10% open with 50% bypassing. Even if the bypass flow varies by +/-10%, then the total bypass variation is 10% of 10%, or 9% to 11% of total. The temp effect of this disturbance can be calculated to assist in making a decision to replace B with a smaller valve. Without more information than you have given, I probably wouldn't make a change of B.

best wishes,
sshep