Vents & Drains - designing pipe 2

[spaulus]

I am new to this forum.

I am looking for information on when to use vents and drains...I assume it is good practice to put a drain valve at the low point on a line...is it also good practice to put a high point vent on a line?

Not every line I see out in the plant has a high point vent or maybe I am missing it.

Does the process of the line determine if a line gets a vent or a drain?

I understand that some times when they have to do a pressure test on a line they have to fill it up with water to test it and then drain it out?

I also understand that after the testing that they sometimes remove the vent valves and seal weld a plug.

As a piping designer is there a rule of thumb on when to use a vent and a drain?

Thank you,
Steve

 

[SNORGY]

spaulus,

Allow me to share my insight on the matter by imparting to you one of my own experiences.

When I worked as an intermediate engineer (field engineer) for the construction company engaged for the Syncrude CAP expansion project in their hydrogen plant area, I was responsible for preparing and getting approval for the hydrostatic and pneumatic test package QA/QC/turnover documents. All drains work better with a vent to avoid prolonged time to draining and, often, violent "burping" that can occur during draining without an adequate vent. The only thing at issue is what form the vent takes; it doesn't always need to be a valve. Sometimes cracking a blind flange open at a high point is perfectly fine.

So, it came to pass that I put together a test package for some Chrome-Moly pipe that, since there was a flange at the top of a 50 foot riser some 3-4 levels up on the pipe rack, I said "Crack open blind to vent for test.".

The field foreman asked me, "Are you sure?".

I said, "Yes, it's a chrome-moly line, let's avoid the field weld.".

The field foreman removed his safety harness, handed me a couple of wrenches, told me that he thought the idea might work, and asked me if I would be so kind as to climb up and do the honours. So I donned the gear, took the tools, and made my way up the scaffold to the 30"-300# ANSI blind flange that I was proposing to crack open to vent for the test.

In about 10 minutes, I came down, took off the harness, handed him the tools, and said, "Vent valve.".

He smiled and said, "Thank you.".

The rule of thumb is, put in vent and drain provisions in the easiest way possible for the field guys and, later, Operators to actually be able to use them.

 

[KernOily]

"Does the process of the line determine if a line gets a vent or a drain?"
Yes. If you can't tolerate a gas pocket forming in a high point in a line, then you add a vent. Example: pump suction line that has a high point. The high point vent can be automated with a small control valve, or locate it in a place where an operator can reach it. Sometimes you have to put in an extension off the valve handle, or run a piece of pipe down to where it's accessible and put the valve there, at the end of the pipe.

We always put in vents and drains at inline instruments (flowmeter, control valve, pH probe, etc.) for maintenance of the instrument. This is because the pressure in the line must be blown down and then the line drained before the instrument can be removed.

You always put in a drain near a pump suction and discharge flange. This goes between the pump flange and the first block valve. This is to drain the pipe when removing the pump for service. Same for compressors, except you would put a vent on top of the pipe to blow down gas pressure. If there's a chance of liquid being in the line then you would put a drain valve also. So it depends on the process, as you said.

So, not every low point has a drain and not every high point has a vent. It just depends on the process, maintenance, and the various needs and things that need to happen during the service life of the pipe.

Now, all the above is for the process design. Vents and drains used for hydrostatic testing of the line after construction are different. We usually don't show those on the P&IDs and on the piping drawings, and we leave the installation of those to the contractor. Sometimes the valve is removed and the thredolet is plugged and seal-welded after hydro is done, as you said, and sometimes the valve is left if Operations wants it left there for some reason. The contractor is best qualified to select the vent and drain locations for hydrotest. THen we can as-build the locations of the hydrotest vents and drains and add those to the as-built P&IDs.

Hope this helps; it's one of those things you learn by experience and you pick up by hanging around and asking questions.