Pressure vessel under negative Pressure

[Paul Arthur]

Hi Guys,
I often use large composite GRP pressure vessels in my water treatment designs and typically they are designed by the manufacturer for a test pressure of 350kPa.
Typically, they would operate at pressures below 120kPa.-
See attached.
Every so often, the water in the top vessel connection goes into negative pressure, typically no more than -30kPa.
The manufacturer says the vessels should never come under negative pressure, but I know that rubbish as they couldn't operate in my water treatment plants if that were true.
My question is, what should I know about this so that I can be better informed when discussing this with the manufacturer.
Thanks
Paul

 

[r6155]

What is the CODE?
Experience in design, manufacturing, and inspection is required to be able to discuss with the manufacturer.

 

[akpipelineengr]

Composites don't do great in compression. Just because it hasn't failed yet, doesn't mean you aren't accumulating damage.

Titan_submersible_implosion

 

[jt1234]

If you have negative pressure on the top, that means somewhere in the top or the pipe line got plugged or stuck while you are draining the water out from the lower section. Check the piping system connecting to the top head to see where is the bottleneck or any valve unnecessarily closed during operation.

 

[SnTMan]

Might consider an air admittance valve...

Regards

Mike

 

[Snickster]

The top of the vessel is the downstream side of the filter elements. If you are getting a negative pressure sometime then it appears at these times you are getting an abnormally high pressure drop across the filter. This could be due to clogging up the filter which indicates possibly the need to regenerate the filter more often. So basically the filter is becoming caked up with the filtered particles and need to be backwashed more often.

Ask the vendor what they think may be causing the pressure drop across the filter to be abnormally high.

Do you have a schematic of your system? Something don't make sense because would not the discharge ig such system be to an atmospheric vessel such as a swimming pool where in this case pressure on downstream of filter will approx atmospheric always?

 

[Paul Arthur]

Thanks for responding guys.
I should have said that this is a water treatment plant for a very large swimming pool.
There is nothing wrong with the system, no blockages, no filter issues etc.
The negative pressure at the top of the vessel occurs when the plant is shut down and the water in the plant tries to drain into the pool. A negative pressure in the pipe supplying water to the pool is created when the plant is shut down.
This only happens when the plant is above water level. When the plant is below water level it can go into negative pressure.
Putting a vacuum break on the filter top relieves the negative pressure but that then empties the return to pool pipework which is not desirable.
Basically, I want to do away with the vacuum break but only if the vessel can safely cope with short term negative pressures of up to -30kPa.
So back to my original question, what should I know about occasional negative pressures at the top of a pressure vessel.

 

[LittleInch]

What you should know is that you are risking collapse of the vessel and the chances increase every time you do it due to the fibres being subvert to reverse forces and breaking.

It's not that different to that Titan sub which imploded after suffering damage every time it went to depth until it went bang.

Composite or GRE type material can fail very suddenly seemingly without warning.

That's aptetty big vessel and 30kPa is a big pressure for negative pressure. It equates to 120 inches water column.

Thank how much force is involved if you submerged that vessel with atmospheric pressure inside and added 10 feet of water....
Q
When the vendor has told you quite plainly it is designed for effectively ZERO inches water column, exposing it to 120 inches is nuts. IMHO.a

 

[Compositepro]

Composites don't do great in compression. Just because it hasn't failed yet, doesn't mean you aren't accumulating damage.

Titan_submersible_implosion

Composites can perform very well in compression. Airplane wings have a balance between tension and compression forces.

 

[LittleInch]

Composites can perform very well in compression. Airplane wings have a balance between tension and compression forces.

Maybe, but then that's their design.

The vendor has been very clear. No negative pressure is allowable. Operating outside of the probably plated MAWP is just not acceptable.

So question to Paul. What is the name plate on this vessel. A picture of it would help.

If it's designed to any recognised PV code then it will or should have a nameplate.