Tag Archive for: check valves

Whether it be a submersible pump you already own or a used pump you are looking to buy, it is important to know the condition of the electric motor. A few quick checks can save you a bunch of time and or money.

The most common problem in a submersible pump motor is water ingress. Whether it be entry though damage to the electrical cable, motor housing or seal system the results are the same. Water and electricity don’t mix well and most often end in what is referred to as a phase to ground fault. When this happens electricity normally contained within the motor windings “leaks” through damaged electrical insulation within the pumps motor and reaches the motor case/ ground circuit.

Fortunately for pump users the slow ingress of water does not immediately result in a catastrophic failure but does cause the winding insulation to weaken, which in time eventually does result in a major motor failure.

If a phase to ground leak has resulted in a catastrophic failure there will be little to no electrical resistance between the power conductor and the pump ground circuit. This is a clear indicator that the insulation at one or more points has completely failed. This type of failure can often be detected with an inexpensive multimeter or ohmmeter. A reading of zero ohms (or close to it) on the lowest setting clearly indicates a major problem. (see below)

If the insulation is only weakened but basically still intact, the applied voltage from a multimeter is not sufficient to detect a problem. To detect this condition a much higher voltage must be applied. Insulation resistance testers generate much higher voltages, up to 1000VDC or more. These instruments are often referred to by the brand name ‘Megger’, but are available from many different test equipment manufacturers.

Higher test voltages can generate leakage currents large enough to yield much more accurate results than a multimeter. Found early, motors with low readings can often be disassembled, dried out, and re-dipped to add extra insulation. This is far more cost effective than a full rewind. Below is a picture of a modern Insulation resistance tester and beside it an older crank-powered one.

Insulation resistance testers often feature multiple ranges, such as 300V, 500V ,750V, or 1000 volts. To apply a test the general rule is to determine the motors normal operating voltage and choose the next higher scale. (eg, for a Megger offering the four voltages listed above, use the 750 volt scale for a 575 volt motor.) Ideally, you want a reading of a few gigaohms or more, but I’ve personally installed 575 volt motors with 750 megaohms resistance to ground and had no problems. This value will decrease as the motor ages.

A SAFETY NOTE. HIGH VOLTAGES IN USE!!! STAY DRY AND DO NOT TOUCH THE ELECTRODES OR ANY PART OF PUMP WHILE TESTING WITH A MEGGER!!!

When looking to test a submersible pump motor, first test the unit with the electrical cable still connected at the pump end but completely disconnected from the power supply. If a fault or weakness is found, separate the supply cable from the pump by opening the pump and disconnecting the motor winding leads from the cable conductors. Check the motor and the cable conductors separately, you may be lucky and have a good motor and only have to re and re the cable.

Just to be clear, the phase to ground test described in this blog will identify 90% of the faulty or weak motors, but only a well equipped rewind shop can perform complete testing of a submersible motor.

Should you need hands on support with a motor problem, or for that matter any pump problem please give us a ring. We are here to help!!

RJ

I often receive questions as to whether a check valve in a pumping system is a good or a bad thing.  Auto closing check valves are often an integral part of a successful pumping system; however, they can also be the root of many system failures. The question then becomes “when is it appropriate to incorporate check valves in a piping system?” Answering this question is often different for systems handling clear liquid or non-settling slurries than it is for systems handling settling slurries, so let’s look at both.

Clear liquid or Non–Settling Slurries

Seven factors to consider when deciding if a check valve should be installed:

  1. If the piping system has a significant elevation change, then back-flow through the pump on system shut down can spin the pump backwards damaging the pump or the pump driver.

  2. If a pump is operating in parallel with other pumps and there is no check valve present, a reverse flow could be induced  when a single pump is shut down or fails. The reverse flow may spin this pump backwards and damage it or the pump driver.  In addition, flow diverted backward through the idle pump would further reduce the production within the overall system.

  3. If the system incorporates a long pipeline, the significant friction component within the system head will be absent on an empty pipe start up. This will cause the pump to operate at run-out conditions as the pipeline is filled. A check valve can limit this problem to the initial charging of the line and thereby reduce possible damage due to cavitation.  It can also similarly reduce issues with the pump driver, if the driver was not sized to handle the high-power draw that run-out conditions create.

  4. Centrifugal pumps have to establish flow through the pump to develop discharge pressure.  Pumps operating against a high static closing pressure on the check valve may not be able to move enough liquid through the impeller to open the check valve and establish flow. This circumstance would necessitate the use of a relief valve between the pump and the check valve and for the relief valve to be temporarily opened every time the pump is started. This of course adds another layer of complexity and cost to the overall system.

  5. Obtaining a check valve with a pressure rating suitable for ultra high lift series systems may be difficult.  For example: a deep open pit mine may have over 1000 ft of back-pressure on the primary pump’s check valve if the check valve at a later lift station fails.

  6. In locations that are subject to freezing weather conditions, dump valves must be installed to drain the line after a shut down when check valves are present. These need to be automated or stringent procedures in order for 24-hour monitoring to be in place.

  7. Auto check valves that allow some degree of back-flow before totally closing can be the source of catastrophic water hammer.

Settling Slurries

The factors to consider for settling slurries are the same as the ones listed above, but the issue of solids settling in the line and the possibility of a plug forming can add further complications.  If check valves are employed, procedures for line flushing must be in place. In addition, dump valves are strongly recommended so lines can be drained of solids should an unscheduled shut down suddenly occur.

My Advice

With so many factors to consider there is no hard and fast rule as to whether a check valve will help or hinder operations. My advice is to:

  1. Look at your system without check valves and decide if any of the factors listed above are a major concern.

  2. If there are concerns, look at installing check valves and make up a list of automation and or procedures that would be practical and needed to address those factors.

  3.  The final step then becomes a cost / risk analysis as to whether the automation and or procedures will appropriately address the concerning factors.

 I trust that this blog will help some of you decide if and how automatic check valves should be installed.  For those readers who were looking for me to simply say “yes or no to check valves”, I apologize!!

RJ