Pump Suction Pressure??

Suction head will be approx 10m. Suction pressure will depend on fluid density.

Fuel Oil Density : 0.82 kg/liter

Approx 0.8 barg (1.8bara).

p = 0.0981 h SG

where

h = head (m)

p = pressure (bar)

Pump Suction Pressure = Pump Suction Vacuum?

NO !

Whether the question is homework or not, it is an interesting question which, judging by the responses so far, and from my experience, is a source of much confusion.

Vacuum is specifically the area of pressure between 0bara (perfect vacuum) and 1 bara (or perhaps 1 atmosphere, to be precise, but let's be approximate for simplicity's sake). You could also call it the area between -1barg and 0barg, which is where the term 'negative pressure' enters the equation.

The term 'Suction' here means the suction side of the pump. Suction also means the movement of a fluid from one zone to a zone at a lower pressure. In fact it has nothing to do with vacuum, but it has everything to do with differential pressure.

In this instance, the OP's sketch indicates a suction pressure well outside vacuum conditions, and I'm sure that I'm not the only one on CR4 that has seen suction pressures in excess of 75bar. So this has nothing to do with vacuum, and the term 'suction vacuum' (like 'suction lift') can be very confusing.

What about the flowrate and the pipe diameter?

Or is only the static pressure of interest?

The sketch indicates 0.5m dynamic head loss.

Initially it would be -ve pressure at suction. Once well primed, the suction pipe would start syphoning and suction pressure would become +ve, say: 10.5 m liquid column.

The sketch is representative only, not to scale, and does not indicate whether the floating suction pipework is above or below liquid level, so we know nothing about priming. It may or may not be required. If it is required, unless the pump is self-priming it might not be capable of priming the suction pipe, in which case there would be no -ve suction pressure anyway.

We have established that the suction head would be 10m, not 10.5m, and this is suction head, not suction pressure.

You would be quite right if your assumptions were correct. You are assuming that an element of suction lift & subsequent siphoning will necessarily occur. Don't be misled by the sketch. Every 'floating suction' system I have ever seen involves a suction inlet below the liquid surface, and a continuous drop from this point to the joint (& tank outlet), with no rise in the pipework at all. So on first usage, the suction pipework is evacuated and the pump is primed as soon as the discharge valve is opened. So the suction pressure is always positive.

Dear all

yesyen has explained it welll. As long as the liquid level is above 2 m, the suction pressure will always be positive during normal operation (not during initial starting-up where the suction line is empty - not filled up by oil). Once the level drop below 2 m, the pump suction will be vacuum.

If we plot the pressure profile along the suction pipeline, the pressure at the oil surface will be 0 atm, then reduces below atm (vacuum) where the vacuum reaches high vacuum at the bend (above the liquid lwevel). Vacuum then drops and finally becomes positive again at the pump suction.

yesyen has explained something reasonably well, but as far as this thread is concerned, it might be irrelevant, or even incorrect, if it is given based on a false premise. Regarding the 'floating suction' arrangement - "…at the bend (above the liquid lwevel)" - you are both under the impression that the suction pipe must rise above the liquid level before falling towards the tank outlet, however this arrangement makes no sense at all, so much so that it would rarely (if ever) be used. A rising suction arrangement would be more complex, but would also require a self-priming pump instead of a standard centrif. (otherwise it will not develop the -ve suction pressure required to raise the fluid). I have certainly never seen such an arrangement, and cannot think of any reasons why it should be done this way, whereas typical floating suction systems, which have a continuously falling suction line from suction inlet to tank outlet, can be seen here and here.

This means that there is no need to 'lift' the fluid to prime the pump & suction pump, so there is no negative pressure on the suction side of the pump, and no siphoning. At initial startup, the suction line fills itself.

Furthermore, in post #6 yesyen also incorrectly gives the suction pressure as 10.5m, when in fact this is suction head, not suction pressure, and should read 10m not 10.5m.

You yourself gave the correct answer to the OPs question in post #14, however this is a repeat of the answer somebody else had previously given in post #3.

Where's Linda Lovelace when we need her!!

More homework?

HEY! Is this this week's challenge question? O wait...

Hope you are asking for NPSH for pump. NPSH = atm. pr.head + (liquid level above pump center line) - pressure loss in pipe line including valve.

If I assume atm. pressure in tank as 10 meter and pressure loss in pipe including floating suction and valve as 0.5 meter then NPSH will be 20 meter.

The OP asks about suction pressure, not NPSH.

Your formula for NPSH is missing one very important element - Vapour Pressure!

Dear Sir

Suction pressure = head differential x density x 9.81 / 1000 kPaG

Head differential = 12 - 1.5 - 0.5 = 10 m

Therefore, in you case:

Suction Pressure = (10 x 820 x 9.81)/1000 = 80.44 kPaG or 0.8044 barG

Hope the above answer meets your question

Sabri Ahmad