Gene Vogel
EASA Pump & Vibration Specialist
Submersible pumps may have any of several types of internal sensors or protective devices. These may include temperature sensors, vibration transducers or moisture sensors. The type of temperature and vibration devices are identical to those used in non-submersible style motors, so only the moisture sensors are of special interest regarding submersible pumps. It is important for technicians involved in the repair of submersible pumps to be aware of their presence in any specific pump they may encounter, and understand the manner in which they function to ensure they work properly after repairs are completed.
Moisture sensor indicators
Moisture sensors along with other internal sensors or protective devices are connected to the machine starter or controller through the power cable or cord along with the power leads; in some cases, they are connected by a cord separate from the power cable. Generally, the presence of control leads entering the pump along with the power leads is an indication that moisture sensors may be present (see Figure 1). One of the important initial steps in disassembling a submersible pump is to remove the cable seal housing (aka pot housing) and inspect the incoming leads.
Since the cable or cord entry is most often at the top of the submersible pump stator and any moisture sensors will naturally be below the stator, it is necessary to disconnect any moisture sensor leads before attempting to remove the stator from the pump. Attempting to remove the stator without disconnecting the moisture sensor leads will result in breaking the sensor leads or possibly breaking the moisture sensor.
Separate sealed cavities
There are two separate sealed cavities in a submersible pump: the motor stator cavity and the seal cavity. Industrial submersible pumps will have two seals with a seal cavity between them that will be filled with some type of barrier fluid, generally oil or glycol solution. Each of these two cavities is subject to possible moisture ingress and may have moisture sensors. In one common configuration, a pair of dual-purpose moisture sensors may protect both the stator and seal cavities.
In other configurations, individual sensors may protect either or both of the cavities. The configuration and purpose of sensors should be obvious from the location of the sensors. But it won’t be obvious from control leads at the cable or cord connection where testing might be done.
The most common type of moisture sensor is a conductivity probe. These are very simply an exposed metal conductor mounted in an insulator. Notice that some sensors are simply a length of tinned lead wire. There may be a single sensor with an electrical potential to ground or a pair of sensors with the electrical potential between them. In either case, the presence of moisture will reduce the resistance in the circuit. The controller will sense the reduction in resistance and indicate a moisture failure. Figure 2 illustrates some common examples of conductivity sensors. The conductivity probes in Figure 6 illustrate a pair of dual-purpose moisture sensors. The sensor circuit is between the two probes rather than from one sensor to ground. The probes are mounted through holes in the bottom of a well in the stator cavity into the seal oil in the seal chamber below.
When moisture collects in the well at the bottom of the stator cavity, the water closes the circuit between the terminals on the top of each probe. If the outer (lower) seal fails and pumpage penetrates into the oil filled seal chamber, that moisture mixes with the seal oil and the resistance between the portions of the probes extending into the seal oil is reduced. Thus these probes sense moisture in the seal cavity or moisture in the stator cavity. Note in Figure 3 that there is a resistor between the probe terminals. The pump controller is set to recognize that resistance value, allowing the controller to detect if a sensor lead is open. To determine a resistor's nominal value visually from color band markings, consult an electrical reference such as the EASA
Electrical Engineering Pocket Handbook.
Float switch sensor
Perhaps the simplest type of moisture sensor is a float switch, as illustrated in Figure 4. The float is magnetic and closes (or opens) an isolated reed switch when the float rises. Unlike other type sensors, float switches are functional only when the pump is setting vertical, but that’s little concern since the normal operation of submersible pumps is vertical.
Figure 5 illustrates an unusual proximity probe moisture sensor. When moisture accumulates as water in the well at the bottom of the stator cavity, the proximity sensor detects the change in density near the tip of the detector.
Evaluating and testing individual moisture sensors during the repair process is a simple matter. Troubleshooting fault indications on an installed pump can be more difficult. In some instances, manufacturers connect various temperature and moisture sensors in series, bringing out only two leads. To test that series circuit, the resistances of individual sensors and trim resistors must be known. Programmed controllers may report a high temperature fault or a moisture sensor fault depending on the circuit resistance change. But a faulty sensor can trick the controller; a faulty moisture sensor could trip a temperature fault or vice versa when the only fault is with the sensor.
Caution during repairs
Repair technicians should be aware of the presence of moisture sensors in submersible pumps and be familiar with how each type operates. Caution is needed when disassembling submersible pumps to ensure sensors are not damaged, and their function should be properly tested during the repair process.
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