Bypass for big sized Control valves 3

[reena1957]

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.

 

[JLSeagull]

Depending upon the fluid properties, temperature etc., consider using high performance butterfly valves. Many double and tripple offset butterfly valves are suitable for use as control valves - thus suitable for use as a bypass valve. The shutoff characteristics are good too.

 

[davefitz]

globe style bypass valves are used on large power plants for the reheater steam bypass dump to condenser block and control valves. They must have forged, not cast, valve bodies, to limit the thermal stress during operation, and also a small warmup nozzle is inlcuded in the body to maintain th ebody at desing conditions to further limit thermal stress. Hydraulic actuators are often used.

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.

 

[JimCasey]

The purpose of the bypass manifold is to allow manual throttling operation if something goes wrong with the control valve. This is a case that justifies charging at a windmill to get the internal standard changed. If it is truly as you say then it is clearly wrong.

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.

 

[JLSeagull]

On lump sum turnkey projects for offshore facilities, it is common not to provide bypass valves. I don't care for that philosophy but it makes sense for parallel equipment where one train can be taken off line to service components such as large control valves. This could be acceptable where four identical trains operate and not at full capacity. One could take one train offline while increasing the duty for the other trains; or operating at reduced capacity.

 

[devaxrayz]

Reena,

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-

 

[itdepends]

What is the purpose of the bypass valve?- is it to control the flow if the control valve fails? to warm up the line during startup situations?

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.

 

[reena1957]

Dear itdepends,
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"

 

[Joesteam]

I personally have mixed feelings regarding bypass valves on large control lines, because they may be left open or leak and cause dangerous situations. I also do not agree to use a gate valve for control just because globes are not allowed.

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).

 

[itdepends]

reena1957- I've just realised- reading through your posts that we don't know what's going through the valve- whether it's steam, gas, liquid, ????

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.

 

[reena1957]

It is the Rich Amine Flash drum LCV feeding to the Amine regenerator at/very near the regenerator feed nozzle with about 10% flashing occuring d/s of control valve. A tough corrosive erosive service with two phase flow. Thanks to all who responded.

 

[tkdhwjd]

First question you must ask before start sizing and determining the types of bypass valve is wether or not the plant can continue to operate with manual adjusting of CV bypass. On my previous projects, that is the first thing I evaluate with the operation representatives.

 

[CHEMANG]

I think if it were a flashing line, the 2-phase flow will be much higher than 10% in case you propose to throttle the control valve bypass manually.

This may eventually lead to serious problem of cavitation.
Am I overstating anything here?

regards,
ANG

 

[73994]

if it is possible to attach a reducer of 20" to any lesser adequate size then Im sure that it will help u...
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...

 

[reena1957]

Hi,
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?

 

[djack77494]

reena,
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