Compressed air pressure vs hydrostatic pressure

[shberry]

TL;DR Summary

Calculation to confirm the difference between a pressure in a compressed air supply hose and the pressure imposed at differing depths of water.

Hi all, I'm new on here and will start off with what I think is a simple clarification. I'm questioning my workings due to previous work being completed by another engineer and thought this was the best place to ask.

Essentially we have compressed air being supplied through a hose, the hose drops to the seabed. To ensure I won't have an issue at the seabed, I'm looking to confirm compressed air supply pressure vs the hydrostatic pressure from the sea. So I have the following...

Air goes into the compressor at 1bara, comes out at 9.5bara. With the various losses in the hoses etc we will be down to 7.5bara. For the sub sea pressure, we are working at 55mtrs, therefore 5.5bar + 1.0bar atmos = 6.5bara. That gives us +1.0bar in the compressed air line which is fine.

The reason for questioning is the previous calculations have used the compressed air out using barg. This is not so good as loosing 1bar on the compressed air means it now only equals the sub sea pressure. To me it doesn't seem correct to use barg on one side and bara on the other? Or am I missing something and the sub sea pressure is actually 6.5barg not bara??

 

[Baluncore]

Your analysis is correct.

If you use gauge pressure it must be referred to air at sea level.
Absolute pressure should be used when the compression ratio of a pump is being considered.

 

[shberry]

Thanks for the confirmation Baluncore

 

[Lnewqban]

... Air goes into the compressor at 1bara, comes out at 9.5bara. With the various losses in the hoses etc we will be down to 7.5bara.

Welcome, shberry!

It is important to be sure that those measured pressures upstream and downstream of the compressor are absolute.
Normally, those values are measured by instruments that use the actual atmospheric pressure as zero reference.
To produce an absolute pressure sensor, the manufacturer of the instrument seals a high vacuum behind the sensing diaphragm.
Another hint: Sea divers use manometric units as meters sea water (msw), which are defined as equal to one tenth of a bar.

If your compressor can actually create a pressure differential of 8.5 bars, it should be able to overcome the discharge absolute counter-pressure of 6.543 bars induced by the atmospheric pressure on the sea surface (1.013 bars) plus the hydrostatic pressure in the 55-meter water column (5.53 bars, considering sea water density of 1025 kg/m^3).

 

[shberry]

Hi Lnewqban, thanks for your reply. To my knowledge the compressors have never been checked using bara equipment. They have been in use for many years so could have been before my time, during my time we've only used calibrated gauges showing 0 at atmosphere. It's only recently that we've got involved with the subsea work. Although we're not responsible for the subsea side of the system, my own curiosity has driven me to check what we're up against.

 

[Lnewqban]

Hi Lnewqban, thanks for your reply.
..., during my time we've only used calibrated gauges showing 0 at atmosphere.

You are welcome.

How did you determine the following values?

Air goes into the compressor at 1bara, comes out at 9.5bara. With the various losses in the hoses etc we will be down to 7.5bara.

 

[hutchphd]

The good news is (unless you need to worry about the strength of containment) the capability of the compressor will be underestimated by using absolute instead of gauge pressure in this context. So a little more margin, right?.

 

[shberry]

I'm rounding the numbers to simplify, but I'm working 1 bara at sea level. We know the pressure we can produce is 8.5barg by gauge readings, therefore 9.5bara.

I've been using this equation for the hose losses which is in an excel spreadsheet using imperial units (oil field norm).

but I have noticed that this doesn't take height difference into account. So maybe I should be using my usual...

 

[shberry]

Just a quick update on this one, disregarding what I said above regarding pressure loss calc.

I've updated my calculations to complete metric using

Which is showing okay along the length of hoses. If anyone is interested I'll post the excel file.