When selecting an electric motive for a pump station, it is authoritative to choose one that can develop adequate horsepower to drive the pump. Often motors are selected to be non-overloading at the end of curve. This means that even at the utmost exponent requirement for the heart, the motive is big enough to drive the pump without overloading. In submersible pumps there are several significant factors that come into play for proper choice. We will start from the top of the motor and work our way down .

Cable Connections:

obviously if the motive is submerged in the fluent the power cable must enter the motor housing at a junction box that is below the fluent level. This is a prime placement for a leak. unlike manufacturers have different philosophies regarding how this connection should be made. many believe that this should be a rigid permanent wave connection with built in strain relief. This frequently has a pack gland around the entrance of the junction box and occasionally a secondary coil cachet to prevent escape. other companies see an advantage to having a quick unplug on the cable allowing the pump to be replaced without the indigence to re-cable the unit to the Motor Control Center ( MCC ). If pump change out is frequent because of the necessitate to de-rag the pump or other operational problems, it may be advantageous to have a pump with a quick unplug. This may be specially truthful if it allows the unit to be changes without the need for calling out an electrician .

Bearings:

broadly in submersible pump designs there are two bearings to be concerned with. The upper bearing that generally designed to support the rotor ( pump impeller, shaft, and motive rotor ) in the radial commission. This bear is allowed to move axially, via a faux pas fit to the caparison. This allows for thermal growth in the rotor as it heats during operation. The lower charge which is broadly responsible for supporting the rotor in both radial and axial load is fixed in place allowing it to transfer the axial loads from the pump into the drive frame .
bearings For pure radial load in pumps, by and large a one row deep rut ball bear ( A ) works well. They are cheap and have more than enough radial load capability for common pump applications.Occasionally double rowing deep furrow bearings ( B ) and roller bearings ( E ) are used when very high radial load is expected but these are often more expensive, and are by and large found on big horizontal pumps.

For the combination of radial and axial loading the unmarried row, deep groove wear can be used but the axial load capability is slightly limited. The doubling row, trench furrow carriage supports much higher axial loading ( frequently 1.7 to 2 times the capability ). When identical high axial loads are expected angular liaison ( C ) and tapered roller ( D ) bearings can be used that have 3 to 5 times the burden carrying ability but can only do so in one direction .
A caution regarding bearing sizes : frequently in specifications I see mean time between failures ( MTBF ) or mean time between repairs ( MTBR ) specified at very eminent numbers ( 60K, 80K, 100K hour ). This leads the manufacturers to install larger bearings to meet the identical high design life requirements. Bearings running in very lightly loaded conditions often cause the bearings rolling elements to skid alternatively of seethe. The skidding legal action causes the lubricant film between the rotating element and the racetrack to dissipate because more vegetable oil is not being drawn in by the rolling action of the bear finally leading to metal to metal contact. This leads to spalling of the bear and finally to its bankruptcy. In my career I have seen many more bearings that have failed due to under-loading than over-loading. The most common cause of bearing failure is actually improper lubrication either from contaminant of the lubricant, or poor preventive maintenance practices .

Oil Filled vs. Air Filled Motors:

oil filled motors offer several benefits, the most pronounce being that because of the much higher thermal transfer capacitance of oil vs. air ( approximately 7X ) oil filled motors tend to run cooler. The anoint besides provides continuous lubricant for the bearings and the windings. It is claimed by some manufacturers that the vibration or start up torsion pulses of the windings causes the insulation to wear finally leading to shorts within the motors. petroleum filled motors are supposed to lubricate the windings and prevent degradation from chaffing during start-up. I don ’ thymine see this as a big trouble unless you are putting an excessive total of starts and stops on your equipment. There are besides studies that claim that oil filled motors prevent moisture from getting into the insulating material on the windings. The insulation is hydroscopic and tends to breakdown more promptly in damp environments .
Air filled proponents tend to focus on the fact that there is a higher amount of drag loss in an petroleum filled motor compared to an air travel filled design. typical estimates range from 1 % to 2 % more loss. They talk about the indigence to sporadically replace the oil, but from my inquiry this is merely commend if the mechanical seals fail and product gets into the bearings. I believe in an atmosphere filled motive the grease in a dirt filled bearing would besides need to be replaced after a varnish failure, so I will call that one a tie. Air filled supporters besides expound about the possible catastrophic affects of an petroleum leak from the centrifugal we must keep in take care that we are not talking about the Exxon Valdez. This is a small come of petroleum and by and large these days, manufacturers use not toxic blends of vegetable oil. It all truly comes down to efficiency and heat waste. If the liquids are always cool and provide enough of heat profligacy, an air out filled centrifugal will credibly work equitable fine. If heat waste might be an return, then I would look identical close at an oil filled design .
If you plan to use your submersible pump in a dry pit application heat profligacy is a major refer. many manufacturers will require that the motive stator be jacketed to help remove the motor heat. The jacket circulates fluid over the outside of the motor helping to dissipate the heating system. These generally come in two forms, intersection cooled and self contained. Product cooling passes some or all of the merchandise being pumped through the crown passing the drive inflame into the pump liquid much like it would be in a inundate application. The problem with this design is that bombastic particulate may cause plug of the jacket ports and lead to a motor failure. respective port designs and configurations have been developed to prevent this, but a frank conversation with your pump seller should happen before you purchase this option. The self contained option uses a separate cooling loop to pass fairly liquid over the motor and broadly does not suffer from the plug problem .

Mechanical Seals:

The job of the mechanical seal is to prevent the liquid you are pumping from leaking up into the bear and motor house. This is accomplished by rotating one extremely flat navy seal face in very near proximity to a stationary face of approximately equal flatness. The faces are lapped categoric to within 2-4 helium light bands of being perfectly flat. In the space shown as the fluid lodge in the diagram below, a little sum of liquid is wicked through the faces and is vaporized by the estrus generated by the faces rotation. There must be liquid in the seal faces to cool and lubricate them. When seal faces run dry they fail promptly, sometimes within fractions of a second. In a typical single seal pump it is the pump fluent that passes through the faces and is vaporized as it enters that atmosphere. In a submersible pump some small units use a single seal to seal the drive from the pump liquid. single-seal-in-a-process-pump

Figure 12: Typical single seal in a process pump

In most submersible pumps the typical seal agreement is a dual cachet in either a double or tandem arrangement .
The dual seal offers the protection of two seals to prevent the pump intersection from getting into the bearings and motor. The tandem placement has both seals facing the same guidance. The buttocks cachet is in the pump fluent ( A ), as the pressure increases in the pump liquid it actually forces the navy seal faces closer together reducing the amount of liquid that passes through the faces. The anoint in the varnish chamber ( B ) is intended to be the lubricant for this lower seal. The upper seal is feed by the oil in the drive or bearing caparison. typical-tandem-dual-seal

Figure 13: Typical tandem dual seal in a submersible pump

With a double seal the two seals are positioned back to back. This has the advantage that both seals are operating in the clean petroleum environment. The disadvantage is that a imperativeness spike in the liquid on the pump side of the sealing wax can cause the seal faces to force open and product can be introduced into the seal cavity. This reduces or destroys the prurience of the oil in the seal chamber, finally leading to a failure of the bearings .

Seal Face Materials:

seal faces can be made from a assortment of seal materials. The most common materials are carbon ( F ), ceramic ( C ), tungsten carbide ( WC ) and silicon carbide ( SiC ). Carbon is a effective seal substantial because it somewhat self lubricate and is fairly cheap. Its bane is abrasives. It is such a soft material that it is easily scratched. That abrasion provides a leak path and the seal fails. Ceramic is much paired with carbon. It is harder than carbon but generally not harder than common abrasives. It excessively is well scratched leading to failures in harsh environments. It besides does not have the mechanical forte of Tungsten and Silicon Carbide ( it flexes under pressure ). It is susceptible to thermal shock ( immediate temperature change ), causing it to shatter. Its primary attribute is that it is cheap, and consequently very popular. Tungsten Carbide is an extremely hard material that has identical commodity mechanical properties coupled with excellent corrosion resistance. The material does highly well in abrasive services. Silicon Carbide, like Tungsten Carbide is extremely hard even slenderly harder than tungsten carbide. It has excellent corrosion resistance and identical good mechanical properties. It is often paired with tungsten carbide as a combination of faces in harsh services .
In most effluent applications it is desirable to have primary coil seal made from hard materials so I would look for SiC five SiC or SiC vs. WC. The upper seal should entirely be pumping oil so a carbon ceramic seal would be a good choice. Some companies offer both seals with hard faces but unless there is a particular cause, I would use F vs. C to reduce the sum of heat generated by the seal and reduce the cost of the pump.

Some manufacturers use proprietary sealing wax designs and proprietorship seal materials. The debate as to whether the proprietary seals are better than the seal commercially available by the seal manufacturers seems to be based more on public opinion than fact. I constantly fear a single beginning for a component because it often leads to higher prices on haunt parts and farseeing potential star times. If you don ’ deoxythymidine monophosphate mind funding your own policy policy by tying up your working das kapital up in you spare parts inventory then proprietorship seals may be a good choice for you .

Moisture Sensors:

thus what if the mechanical seals fail ? How would you know ? On distinctive summons equipment the puddle under the pump is your first warning. With submersible equipment there is no such ocular index. Most manufacturers of submersible equipment have at least one moisture signal detection device in the pump. I will not attempt to explain all of the variations in technology that this moisture signal detection detector may employ. Most are time tested engineering that will give you an indication that you have moisture in a part of the pump that should not. I do believe the location of these sensors is authoritative. Some manufacturers mount the detector in the pit between the elementary seal and the secondary navy seal. If you primary seal leaks the secondary seal prevents the fluid from getting to the bearings and drive where it can cause wrong. Most Manufacturers using this method acting additionally state that the repair of the pump can then be scheduled for a commodious time, the pump does not need to be shut down immediately ( just very soon ). I believe that this is the proper position for such a device. early manufactures locate the detector in the motor cavity which means that you already have liquid in the bearings and the motor when the detector alerts you that you have a problem. This would then require an immediate closure of the unit of measurement to prevent further damage. ( This warn seems a little late in my opinion. )

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