Suction side pulsation
The answer - 2 do's and 2 don'ts
"Started" for reciprocating pumps means each suction stroke.
Local suction volume elasticity is an essential.
2. Do not insert orifices or restrictions they will cause wave rebounds at a higher frequency, and cause frothing.
Essentially, liquid is not like a string; it can not be pulled or sucked; there is insufficient "surface tension" between the molecules, for it to retain a continuous column form if any suction force is applied.
Liquid moves because of a higher force - or pressure - at its’ source than at its destination. Fluid flows into a pump because there is a lower pressure in the pump than where the fluid is coming from. That is to say that it is "pushed" not "pulled - or - sucked". Ideally the "push" comes from height of supply or "head", (approx. 2.2 feet high water produces 1 psi) plus ambient air pressure say 14.5 psi a.
Nearly all liquids contain dissolved air or gas, unless they have been subjected to a high vacuum, and then been stored in a tight container. At ambient temperature the amount absorbed can be very considerable depending on the gasses, so of which are more readily absorbable, and depending on the liquid, some of which can contain more than others. The quantity of gas is in addition to "entrained" air - or micro bubbles in the liquid.
When any pressure is reached, that is less than the pressure at which the gasses were absorbed, it comes out of "solution" and appears as bubbles. This is why it is unwise to put a pressurized gas blanket or aka a "pad" in the top of a feed tank. Whilst the additional force may help to accelerate the liquid supply in the feed line to the pump at the instant of start-up; the pad gas coming out of solution will reduce the "prime" and cause a fall in volumetric efficiency. |
