Why do we have to protect our pipelines against negative pressures?

I was fortunate enough to have a mentor, Stewart Hogarth, (I mention his name in honour, as I was proud of his achievements with me as a student) a Civil Engineer from Scotland. Stewart always maintained that low pressure conditions in pipelines are as dangerous as high pressure conditions.

My thinking was – how can that be possible? I was on site when we were busy with the commissioning of a pumping line to a reservoir at a higher altitude, and of course the pump start-up was fine and we got it running at the design static head, but as the altitude control valve to the inlet of the reservoir opened, we lost the delivery head and the control valve closed – refusing to open again.

After a whole morning of thinking and trying to resolve the puzzle, I decided to boost the line pressure through the test pressure pump, and ended up with a dismantled coupling joint, and some leaking joints elsewhere. This after the inspector had just cleared the pipe line for passing the test successfully. The result was a lot of repair work and a re-test some days later – not having understanding of the problem.

Stewart then explained that by having a high pressure in the pipe (due to the test pump boosting the operating pump pressure), I managed to exceed the test pressure and with the seals having been under negative pressures due to oscillations, and as I never allowed for a vacuum breaker, the damage was done.

All pipelines for water transportation is designed to have positive pressures and contain that up to its pressure rating, and seals of joints and valve seals are all meeting the pressure requirements, but does not hold vacuum. The simple reason is that seals are not designed to operate in both directions. There are some seals, very costly but effective in cryogenic applications, but will not be cost effective for water distribution pipes.

We are also very understanding about double acting air valves which will break vacuum as well, but we must remember the following; All sizing programs for air valves, are principally based on selecting the vacuum break capacity of the valve – not the air release capacity. The reason being is that specific applications do not require air release, but always vacuum breaking. Also, the air release requires operating functions or components that will restrict the vacuum break capacity, like non-slam devices, and reduced port valves. At the same time it is not wrong to use double acting valves, but always ensure that their capacity for vacuum breaking is sufficient. (The size is the important part – always select on the CV factor.) Aquestia is a company that takes pride in manufacturing the ARI model VB 060 vacuum breaker which is full port without any restrictions in the vacuum break flow path.

Floquip (Pty) Ltd,

+27 (0) 82 677 5374,

hannes@floquip.co.za,

www.floquip.co.za