Bypass for big sized Control valves
Bypass for big sized Control valves
(OP)
Hi,We are designing one of the biggest units which has control valves of size 20inches and above. The in-house standards do not allow globe valves on the bypass as they are very big but only allow gate valves which will not help in throtlling the flow. One solution suggested is to go for two 50% size control valves in parallel with isolation block valves. Any comments/ideas/suggestions/best practices please??? Thanks in advance.





RE: Bypass for big sized Control valves
RE: Bypass for big sized Control valves
I remember a comparison of a 20" cast EHD globe valve, 2500 lb special class, being compared against a similar capacity 14" forged angle globe valve, and the 20" cast valve weighed 20,000 lb vs the cast valve at 4200 lb, and the wall thickness of the cast valve was 13" thick in the crotch region, while the forged valve was only 4 " thick. As the thermal stress varies by the square of the wall thickness, the forged valve was clearly superior in terms of minimizing the fatigue damage caused by sudden temperature upsets.
RE: Bypass for big sized Control valves
THe isolation valves in the bypass can be gate valves, but the actual modulating valve should be selected to be at least temporoarily suitable for throttling the flow.
And it takes 4, half-sized valves to equal the capacity of a full-sized valve. Capacity goes up as the square of line size.
RE: Bypass for big sized Control valves
RE: Bypass for big sized Control valves
I always consider that the gate valve is not suitable for throtling because of the erosion on the disc. Because of gate valve is less restrictive than globe valve, it need less opening (leads to higher velocity) for the same flow compare than globe valve.
However by-pass is rarely use and used in short time not permanently (it should be only in control valve fail condition), so the use of gate valve as throtle valve may not lead to much erosion and cause it leak when it is in total closed position. Depending on how frequent the by-pass use, i think the use of gate valve is not a problem, i've seen it some time (in house-standard may refer to past experience)
If by-pass valve use in start-up for preheating/precooling, you might want to consider more smaller additional start-up bypass (4" or 6")
CMIIW
-rayz-
-rayz-
RE: Bypass for big sized Control valves
In large steam lines- a bypass (or warm up valve) is a better way of heating/pressurising up a large 50bar steam line than openening a 20inch control/block valve.
RE: Bypass for big sized Control valves
This is a bypass required for maintenance. Further, do we add one more bypass valve (small size) for opearting the hand wheeled CV or the big bypass valve, especially "when the valve(both CV and bypass) is closed and we want to open it"
RE: Bypass for big sized Control valves
For very large control valves, going with 1/3-2/3 control is a valid solution, with isolation valves so that one of the lines can be taken out of service. We have also used manual handwheel overrides on the control valves so they can be used manually should something fail. AND, it is also typical to get big gate valves with a small bypass installed in the body of the valve for warm-up (especially in steam service).
RE: Bypass for big sized Control valves
Realistically - yes bypass valves can be used to allow control valve maintenance- but you need to determine what the risk is if the manual valve is not closed or not adjusted in a hurry.
Regarding the type of valve- as JLSeagull noted- it depends on the fluid properties- but I'd agree with him that (if feasible) a butterfly valve is far superior to a knife gate valve for the purpose of a maintenance bypass.
Further information on the fluid going through the control valve, it's purpose and any impact variations in it's flow may have on upstream/downstrem equipment is required to provide more meaningful answers.
RE: Bypass for big sized Control valves
RE: Bypass for big sized Control valves
RE: Bypass for big sized Control valves
This may eventually lead to serious problem of cavitation.
Am I overstating anything here?
regards,
ANG
RE: Bypass for big sized Control valves
or if possible to make couple of 90 degree bend in the line so that it will produce enough pressure drop that only a gate valve can handle the 20" flow of fluid...
RE: Bypass for big sized Control valves
Thanks for all your suggestions. We will not be able to control with a gate valve on bypass. We are now proceeding with two 50% CVs in parallel which will open progressively (one after the other) and also a gate valve bypass for 50% flow(normally closed when both CVs are available and open when one of them is under maintenanace). This way we will have a control even when one of the CVs is out but still have 100% flow. Any comments?
RE: Bypass for big sized Control valves
I like your solution. It seems like you'll have reliable control, 0 to 100%, even with any single valve totally out of service. If the service is extremely critical and you expect very long periods between shutdowns, you may even want to have isolation valves around your gate valve. Then you could replace any of the three possible active valves. Of course, there would be a price to pay for that flexibility.
I would also second joesteam's suggestion to use manual handwheels for your large control valves. For larger valves, they are a relatively inexpensive option that can effectively address many (most?) failure scenarios.
Doug