From time to time you are likely to have some troubles with rotary pumps. The most common causes of trouble are (1) the system fails to build up the required pressure, or (2) the pump fails to discharge fluid. When these troubles occur, pro-ceed as follows:

1. Stop the unit.

2. See that all valves in the pump suction lines are open.

3. Check the packing of all inlet valves and manifold valve stems to ensure that no air is being drawn into the suction piping.

Figure 13-5.-Positive-displacement, double-screw, low-pitch pump.

4. Check the pump shaft packing for air leakage into the pump.

5. Check the spring case and the inlet and outlet connections of the discharge relief valve to ensure that no air is leaking into the pump suction.

6. Start the pump again. When it is up to the proper speed, read the suction gauge to see if the pump is pulling a vacuum. If a low vacuum (5 or 6 inches of mercury, or less) is indicated, air is probably leaking into the pump casing. If no vacuum is shown on the suction pressure gauge, it is possible that the pump is not primed. (This should rarely occur once the pump casing has once been filled.) If the system still does not build up pressure, close the discharge valve gradually, and note the pressure gauge at the same time. If the pressure increases, an open discharge line is indicated. If the pressure does not increase, open the discharge valve and close the suction valve.

If the pump is in good condition with close clearances, a vacuum ranging from 15 to 25 inches of mercury (in.Hg) should be indicated by a vacuum gauge connected to the inlet of the pump.

CAUTION

Do not operate the pump any longer than necessary to get a gauge reading; the pumping elements depend on a constant flow for lubrication.

If a higher than normal vacuum is indicated, there may be an obstruction in the inlet piping or the suction strainer may be clogged. If the pump is noisy, mechanical defects such as misalignment or a bent shaft may be indicated.