Liquid Dynamics International Ltd. - Piping System Analysis / Piping System Simulation

LDi - P14

Whether Liquid Dynamics International “PulseDoctor” on site services, you, or a local oscillograph user captures the shock and pulsation "signatures", it is essential that readable suction side plots be taken first and interpreted.

A combination of these suction system samples will be shown by a plot.
30 years experience studying plots of "pressure over time" is an advantage.

1. Suction pipe signature, where a pump momentarily lost prime, or was started too fast for the
suction pipe length.
When a pipe is oversized, yet not so large that it becomes an amplifier, you will see a multiple rebound of each pressure change. The interval between each repeat of the pressure signature is related to the pipe length and pressure wave velocity, depending on "softness".
The repeats will be equivalent to 1400 meters per second + or -, and the length round trip.
Where the height of each repeat decreases, as sample shown here, the pipe is slightly dissipative. It has a little delta P (Pressure Drop). Good, as it prevents resonant amplification.
The desolution of air, having the same effect as reaching vapor pressure, is characterized by a flat bottom to the pressure signature.
This indicates a pressure below, which it cannot fall. Often called "cavitation".
When pump is not Centrifugal, Vane, Lobe, Gear, Progressive Cavity, Screw, etc.
Note: If the suction supply system is designed to feed a reciprocating pump, a signature like this may be seen for every stroke. Do not increase the pipe size or the supply pressure force, put a supply close to the pump.

2. Suction pipe pressure signature, where a check valve is often before the pump to maintain the
prime. A check valve or "foot" valve is never simply held open by the viscous drag of the liquid.
Check valves modulate between nearly closed and near full open. They often "chop" flow into pulses.
Valve beat fluctuations are characterized, particularly when they are integral with a diaphragm or plunger pump, by a different interval between beats, a.k.a. "frequency", different pressure level, and normally with a rounded top and rounded bottom.
When pipe is long and size gives a good dP, that "scrubs" pulsation with "reynolds action" reverberations will not be seen, they will have been dissipated against the pipe wall.
The mass of liquid is always greater than the weight of a check valve moving element, and the force even of a check spring, is always less than the pressure in the liquid. The supply column to a pump always excites the movement of the suction check; it then behaves according to its own response characteristics and the pounding of the liquid.
When a supply system is broken into slugs and voids by air out of solution, check valves are hammered and the pulsation becomes as violent as a relief valve bouncing.

3. There is a tendency in the trade to ignore valve action and the chopping of flow, and to diminish the importance of correctly selecting pipe size, then to consider everything in terms of "acoustics". By acoustics the experts seem to mean only the results of pressure activity that is reflected from the closed or open end of a system, and from direction changes in the conduit.
In acoustics, all that is relevant is the speed at which pressure changes travel from place to place.