Pipeline Pressure Restfill

[Tuga29]

Hi all. This is not my area and that's why I request your help.
In a oil product pipeline what should be the restfill pressure? The restfill is done with the product that specific pipeline transports (ULP, ADO, HFO or JET). My concern is regarding some grade of corrosion prevention.
If there is in your opinion an optimal restfill pressure, is there any specification or standard?
Thank you.

 

[LittleInch]

I've been in pipelines 35 years and not come across "restfill" before. I guess you mean shut in pressure?

Minimum pressure is that required to maintain at least say 5 bar at the highest point to avoid a vacuum forming.

Max pressure is the MOP, but usually somewhere in between. Also depends on temperature of product when it goes in. The higher the difference between product temperature and ground temp the higher the pressure to compensate for pressure falling as the temperature falls.

I don't understand the reference to corrosion. Can you explain more?

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[Tuga29]

Yes, shut in, we call it resfilling pressure.
Concern is that pressure being low, leads to the creation of vapor pockets that will collapse creating erosion on the pipeline wall.

 

[1503-44]

If you want to maintain a liquid in a liquid state, keep it above its maximum vapour pressure.

A black swan to a turkey is a white swan to the butcher.

 

[LittleInch]

won't get erosion, but you could get more damage due to very high pressure spikes when that vacuum collapses when you re-start. These short spikes can easily be 2-3 times MOP. Do that too many times and you find the weak spot in the line.

How you get erosion when their is no movement of fluid? Or corrosion for that matter when the pipe is only full of vapour?

So to reiterate - the shut in or restfill pressure should be somewhere between min pressure and MOP.

Operating company I used to work for always closed the downstream end valve first, then kept pumping for a few minutes sometimes to pack the lines up to 30-50 bar or when the pump tripped on low flow(!)

but there is no "standard" or spec for this, simply company operating procedures or may be written in the original project Operating and Control philosophy. That's where I write this sort of guidance to the operators. Now whether they read it or not I'll never know, but if the line breaks later on I can say I told you what to do.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

He seems to be talking cavitation erosion, but that isn't specifically related to shutin/shutdown pressure.

Other than that, corrosion doesn't particularly care if the fluid is a liquid or a gas.

With a product pipeline, if you shut down while operating, you Must maintain pressures everywhere along the pipeline well above each product's vapour pressure. If you don't and one product vaporizes, you can find your pipe full of a well mixed combination of off-spec crap that will need to go back to the refinery. That happened at Site 7. They made a big load of gasoline/jet in the tunnel when they shut down with an interface located at the pipeline's high point and the pressure during the shutin dropped below the vapour pressure of the gasoline. They wanted to know how 2,000 bbls got into the pipeline.

A black swan to a turkey is a white swan to the butcher.

 

[LittleInch]

Ha. Just a well I know what site 7 is .

But yes I forgot about having an interface in the line when you shut in. My ROT was that for every start stop added 10% to the interface length and if you let it go below vapour pressure all hell will break loose.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Tuga29]

So, how was that off-spec crap formed?

 

[1503-44]

The gasoline vapourized and the bubbles displaced into the jet fuel, which being uphill from the gasoline didn't help, and that forced the jet to run downhill along the bottom of the pipe into the space previously occupied by the gasoline bubbles. Gas bubbles went up, jet down, mixing into any liquid gasoline it encountered along the way. When they started the pipeline again, the pressure rose, collapsing the gasoline vapour back into liquid and it all made a big mess. Missed a zero on the 2000 bbls. It was nearly 20,000.

It's never good to stop a multiproduct line. Even if you keep the pressure higher than VP, density differences at any interface between batches quickly start flowing up or down and the batches try to displace each other. They will even do it on horizontal pipe, but if they are on a sloped pipeline, they will move surprisingly fast and make a real mess.

A black swan to a turkey is a white swan to the butcher.

 

[Tuga29]

In this case it’s not a multi product pipeline. But my question was raised because operator is using a restfill pressure close to product maximum vapour pressure or even lower. They don’t take care of the pressure.

 

[LittleInch]

Do the operators report any loud banging noises when they re-start as well?
Or evidence that any of the pipework has "walked?".

Depends a little on how long the pipeline is but that's poor practice IMHO. How long / big is yours?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Usually not a good idea. Some products have a large change in vp with changes in temperature.
Best to not let it vaporize.

A black swan to a turkey is a white swan to the butcher.