In a Control Station Assembly/ Control Valve Manifolds we try to put the by-pass valve as near to upstream side as…

The reason for putting the bypass valve in the upstream is to avoid the ded leg in the line but that depends upon the configuration of the control valve manifold. There are different methods of placing the bypass valve in the CV manifold. CV manifolds up to 8" sizes we can have the bypass valve on the top by creating a room for the CV maintenace. Above 8" it needs to be provided in the vertical leg thereby creating a deadleg which normally is not preferable. Although if the client approves then this can be managed.

In my opinion this is an example of "Fuzzy Logic".
"Fuzzy logic is a form of multi-valued logic derived from fuzzy set theory to deal with reasoning that is approximate rather than precise."

Statement #1.
"In a Control Station Assembly/ Control Valve Manifolds we try to put the by-pass valve as near to upstream side as possible." This is the proper placement for the location of the throttling (bypass) valve in every case. If there is an exception to this I have not seen it yet.

Statement #2.
"Somebody told me we do this to avoid stagnant flow in the upstream side." This is not a valid reason. There are three basic types of commodities inside a piping system. There are Vapor (or Gases), there are liquids and there are Slurrys. The only case where this might apply is when the commodity is a "slurry". In the case of a "Slurry" you would use an entirely different type of valve and Control Valve Manifold configuration. In the case of the liquids there is the possibility of some sediment settling out and build-up in the stagnant area, however that is not reason enough to locate the bypass valve near the upstream side. In the gases there will always be condensables that drop out of any vapor flow. Condensables will drop out because of a drop in temperature, a drop in velocity or drop in pressure.

Statement #3.
"But if this is the case then there will be stagnant flow in the downstream of by pass valve." This is true and it does not cause any trouble in most cases. If the commodity were a slurry or it was inverted so as to create a low point then it could be a problem.

Statement #4
"The reason for putting the bypass valve in the upstream is to avoid the dead leg in the line but that depends upon the configuration of the control valve manifold." This is the same as Statement #2

Statement #5
"There are different methods of placing the bypass valve in the CV manifold. CV manifolds up to 8" sizes we can have the bypass valve on the top by creating a room for the CV maintenance." This statement is okay.

Statement #6
"Above 8" it needs to be provided in the vertical leg thereby creating a dead leg which normally is not preferable." This statement is wrong on many issues and should never be done.

Statement #7
"So I want to know the correct reason for putting by-pass valve near to upstream."
Answer #1
The primary reason for placing the Control Valve Manifold (CVM) Bypass valve in the horizontal bypass line and close to the up-stream side of the bypass is cost. In many (if not most) CVM's the control valve itself causes a change in the line conditions. This change will be reflected in the difference in the pressure between the upstream side of the manifold and the downstream side of the manifold. In many cases the material will also change from a more expensive material on the upstream side to a lower cost material on the downstream side. By placing the bypass valve close to the upstream line side then you limit the higher priced material to the Tee, the Flange, the valve and the second flange. You do not require any other higher cost pipe or fittings. The cost difference may seem small for a single CVM but over the whole project with hundreds of CVM's it does add up.

Answer #2
A secondary reason for locating the Bypass valve close to the upstream line relates to the vapor or gases. Steam is a vapor or gas and it has condensables, lots of then. The horizontal location allows the condensate ahead of the valve to free drain.

Answer #3
The company where I worked for so many years had a "Design Standard" showing 10 or 12 different configurations of Control Valve Manifolds. Some were for small bore Screwed piping, some were for small bore Socket-weld piping and the balance were for large bore Flanged piping. Some of the configurations were the standard "upstream from above with downstream to above" Others showed the upstream from low horizontal or from below as if the CV was on an elevated platform. Others showed different configurations for the downstream side. As the Lead Design Supervisor on a project I reviewed that Company Standard at the beginning of the project and selected what I thought fit that project. This was then reviewed with the proper Client Representative for approval. The drawing with the "Approved" configurations was distributed to the designers for use on the project. This achieved a level of consistency across the whole project because it eliminated the possibility that every designer would come up with his or her own quirky designs. I viewed it as part of my Design Quality Control.

The reasons will be more than one to put the Bypass valve closer upstream than downstream

1. Cost is one of the driving factors alright but then there are many situations where only pressure is reduced or flow is throttled. This results in rating change (material does not). And still you need higher rating flange on the lower side to meet the valve face. So the weights and thereby cost is not the only limiting parameter for putting it closer upstream.

2. The bypass valve remains normally closed and is opened only when CV is not operating. So the fluid won't be circulating (stagnant; dead leg). This can be problematic especially in case of slurry. (Of course for handling slurry type services butterfly valves are preferred but it is ON / OFF valve and poor controller). So bypass tapping shall be

[i]a. Vertical Tapping on Horizontal line for slurry so that solids do not enter the bypass.

b. For condensible vapor or gas bypass shall be tapped horizontally on vertical line so the condensate keeps draining through the drains.[/i]

3. And as far as, the downstream stagnation is concerned. The filled line allows easier opening of valve (It matters if the pressures upstream / downstream differ largely) and it won't enter the valve anyway as it is downstream. Upstream as you've seen in point 2 is altogether different case.

Thanks to you all for responding.
It will help a lot

Siddharth Jain