14 replies to this topic

[trungphu]

Hello everyone,

 

I have three why-questions regarding valves as follow:

 

1. Why do some pressure safety valves have a bypass, while the other don't?

2.Why is a butterfly valve usually employed at the inlet of a heat exchanger?

3.Why are two-valve in series used for pipe of drain system?

 

I would be thankful if anyone here help me out with these problems.

 

Best regard,

 

Trung Phu

Edited by trungphu, 07 March 2015 - 08:36 PM.

[fallah]

Trung Phu,

 

In brief:

 

1- Because considering by pass for PSV isn't mandatory; some companies consider but some of them don't...

 

2- It's not a general practice to consider BF valve at the inlet of the exchangers...

 

3- This is a sort of double block to minimise the leakage and also for the ease of draining operation...

[breizh]

Trung Phu ,

 

On the utility side ( cooling water ) I get use to see BF , reason being the cost versus the hazard (most probably) and the cheapest way to split the stream between several HX .

hope this helps

Breizh

[trungphu]

Fallah :

 

Thank you so much for your answer. However, I am still wondering what the purpose for using bypass for PSV is?

 

 

Breizh :

 

I want to add one thing. I get used to see a BF are often used at the inlet in the tube side of a heat exchanger when there is a bypass from the inlet to the outlet going past the heat exchanger.

Edited by trungphu, 08 March 2015 - 04:45 AM.

[fallah]

Trung Phu,

 

The purpose of using by pass for PSV is mainly depressurizing the vessel, on which the PSV is installed, in maintenance case, also forwarding the purge gas toward the flare network...

[trungphu]

Thank you so much, Fallah. That's very helpful.

[trungphu]

By the way, I am wondering why a check valve is usually followed by a gate valve?

[fallah]

trungphu,

 

It's not a general rule having a gate valve downstream of a check valve; a check valve prevents reverse flow and a gate valve makes isolation in a line. If in any case, failure of a check valve to prevent reverse flow will result in severe consequences, it might to remove such consequences a gate valve would be considered at downstream for line isolation of course with the expense of having no flow situation...

 

Also, if a check valve needs to be inspected periodically it might a gate valve would be considered at downstream for required isolation...

Edited by fallah, 18 March 2015 - 12:23 PM.

[trungphu]

Thank you Falllah! That makes me clear.

[trungphu]

In industry, I find some control valves have a bypass valve, the others don't. My question is when a control valve need a bypass. I hope to receive the answer from your practical experience.

 

Regards.

[fallah]

trungphu,

 

http://cheresources....-by-other-valve

Edited by fallah, 28 March 2015 - 09:41 PM.

[trungphu]

Mr. Fallah, Mr. Breizh

 

I read an operating instruction and it states that a butterfly valve is located in the outlet of the heat exchanger in order to ensure enough of a pressure drop for the bypass valve on that heat exchanger to function during plant operation at low capacity. The butterfly valve is provided with a mechanical stop mechanism to keep the flow through heat exchanger from being blocked completely.

 

I hope to receive your valuable comment for the above statement.

 

Regards,

[fallah]

trungphu,

 

Lack of adequate clearity in the statement...but appears it wants to say a mechanical stop is to be provided in butterfly valve at the outlet of heat exchanger to prevent the valve being fully closed in order to keep the flow through the heat exchanger even in, let say, turndown conditions...

[trungphu]

Fallah,

 

I agree with you about the mechanical stop but why we need a butterfly valve here? Is it used for reduce the pressure in order to support the bypass valve? In this case, the bypass valve is used to control the temperature of the outlet.

 

Regards,

[fallah]

trungphu,

 

I don't know anymore about the system you described, but using butterfly valve there might be due to economical matters consideration as for the bigger size the price difference between a butterfly valve and e.g. a ball valve is substantial and the butterfly valve is cheaper. Also butterfly valves are usually preferred in non critical services such as cooling water systems.