Small movements of the pressure gauge pointer are in most cases not a machine fault but the measuring chain itself amplifying a pulsation. They cannot simply be ignored either, however, because some of the movement is in fact an early sign of blockage, leakage or cavitation. The key to the judgement is to separate the movement that can be ignored from the movement that has to be investigated.
Three sources of the movement
The first source is a badly located pressure tapping. If the tapping is opened directly below the pump outlet, immediately beside an elbow, or directly after a valve, the pressure pulsation is transmitted to the dial unchanged and the pointer follows it. The flow in the pipework has pulsations of its own; the amplitude is greatest close to the source of the pulsation and the further away the tapping is, the smoother it becomes.
The second source is the absence of damping. The pulsation needs some travel to reach the dial from the tapping, and if that length of pipework is straight, short and without a damping element, the pressure wave passes straight through. In industrial practice a damping valve or an orifice is fitted at the tapping, or a surge vessel is installed.
The third source is the air in the hose and the condition of the hose. As soon as there is an air bubble in the hose, the bubble is compressed and expanded under the pressure and it amplifies the pressure pulsation several times over, which shows as a violent movement of the pointer. In addition, when the hose is too soft, is kinked or is flattened by suction, the volume of the line itself changes, and that produces pressure fluctuations that do not actually exist.
How the pressure tapping should be chosen
- Take it on a straight length of pipe far enough downstream of the source of the pulsation, the distance being generally taken as a certain number of multiples of the pipe diameter, so that the pulsation decays naturally first.
- Avoid opening it directly downstream of an elbow, of a valve or of a reducer; the flow pattern at these positions is disturbed and the pressure is least stable there.
- Keep the tapping pointing upwards as far as possible, so that air bubbles float out on their own and less air accumulates in the line.
- Keep the pressure pipe as short and straight as possible, to reduce the additional fluctuation caused by a change in volume.
Several simple ways of adding damping
The cheapest is to fit a damping valve: a valve with an adjustable orifice is connected in series between the tapping and the dial, and it blocks out the high-frequency components of the pulsation. The second is to install a surge vessel, using a small-volume container to spread out the energy of the pulsation. The third is to lengthen the pressure pipe and fix it in right-angle sections, so that the pressure wave loses part of its strength at the bends. The hose must be pressure-rated tubing, fixed reliably at both ends with clamps or threads so that it cannot kink or be flattened, and where necessary a small vent hole can be opened at the high point of the hose to let the air out, or the bubbles are simply purged before the hose is used.
Which movement can be ignored
There are three criteria for the judgement: the reading fluctuates regularly within a very small range, and the frequency of the fluctuation matches the pump speed or the rhythm of the stroke; there is no other abnormality apart from this, no leakage, no abnormal noise, and the motor temperature and current are both normal; and the pointer of the gauge is neither sticking nor failing to return to zero. Movement of this kind is the transmission of a pulsation and it can be improved by fitting damping or by moving the tapping.
The other way round, the following are not pulsation and have to be investigated: the reading drifts steadily in one direction and does not come back; the pointer jumps suddenly to a point and then stays there; the amplitude of the fluctuation clearly increases; and leakage, abnormal noise or an abnormal vacuum level accompany it.
| Reading | Most likely cause | How to confirm | Action |
|---|---|---|---|
| Small regular movement, the frequency matching the speed | The pulsation in the pipework is transmitted directly to the dial | Move the tapping to a straight length of pipe and see whether it improves | Fit a damping valve or lengthen the pressure pipe |
| Violent movement with a distinct tapping feel at the dial | Air in the hose, or the hose is flattened by suction | Stop the machine, remove the hose and check for trapped air and kinks | Purge the air, replace the hose with rigid pressure-rated tubing and fix it |
| The reading falls steadily in one direction | A blockage or a leak downstream, not pulsation | Compare the other pressure gauges and the change in flow | Investigate in the direction of the leak or the blockage |
| The pointer does not return to zero or the reading is high | A fault in the gauge movement or in the pressure pipework | Compare against a reference gauge | Replace the pressure gauge and clear the pressure pipework |
Safety preconditions before removing a gauge
Before the tapping is moved, a gauge is changed or the pressure pipework is cleared, the machine must be stopped, the pressure released and the power disconnected. Removing a gauge under pressure lets the residual pressure on the high-pressure side discharge straight out and can injure people. After refitting, retighten the gauge and check again for leakage; where the sealing face is scratched, the seal should be replaced rather than simply smeared with sealant.
Small regular movement of the pointer is a measurement problem; a reading that drifts in one direction is a machine problem. Establish which of the two it is before touching anything.