NET POSITIVE SUCTION HEAD OF CENTRIFUGAL PUMP

Net Positive Suction Head ( NPSH ) is extremely important to the operation of a centrifugal pump. Factors that affect NPSH are atmospheric pressure, suction line friction loss, elevation, fluid temperature and SG.

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MOTOR INSTALLATION AND TROUBLESHOOTING

When replacing a motor, its exact dimensions, as well as speed, hp, and torque characteristics should be determined and duplicated if the same performance is desired. When replacing a motor, the entire system should be inspected for internal and external degradation. Neither the motor mounting nor the mechanical coupling should exhibit signs of wear.

The power supply and connections should not be damaged. These should be frequently checked for proper frequency, voltage, and voltage balance between phases. Poor and broken connections of one of the supply lines are a major cause of voltage unbalance. Overload relays for each phase should protect against extreme (greater than 5%) voltage unbalance.

Misalignment between the motor and the driven machine (e.g., centrifugal pump ) can cause bearing failures and shaft breakage. Excessive vibrations frequently indicate misalignment. All motor feet must be fastened to a flat, preferably machined, surface. Otherwise, the frame can bend when the motor is tightened down, which twists the motor frame and causes misalignment. Care should be taken to evenly tension mounting bolts. If torque values are specified, follow the manufacturer’s recommendations. The mounting should be inspected frequently. If necessary, retighten bolts with the proper torque. If vibration is detected, one or more of the motor feet may have to be shimmed. A few thick shims are preferable to many thin shims if it is necessary to align motor and machine (pump) shafts. Misaligned couplings create bearing loading in both the motor and the machine, causing high-speed distortions and also increasing power consumption.

Motor should be greased using manufacturer’s specified greases in concert with manufacturer’s specified lubrication frequency and quantities (sealed ball bearings cannot be lubricated after manufacture). Relubrication is necessary to replenish grease that has broken down by oxidation or been lost by evaporation and centrifugal force.

Inspect and keep cooling and ventilation vents clear of obstructions.

If a motor burns out, the windings should be inspected for signs of single phasing, short circuiting, overloading, and voltage unbalance. Any cause of winding damage should be identified and corrected. If a motor burns out, the circuit supplying the voltage should also be inspected for broken or shorted wires, burnt contacts, or voltage unbalance.

CENTRIFUGAL PUMP AND STANDARD DRILLING EQUIPMENT

Hoppers, mud guns, desanders, desilters, degassers, and triplex pumps requiring supercharging all have one thing in common: they require 76–80 feet of inlet head to operate as designed. Exceptions do exist, and the equipment manufacturer should be consulted. This simplifies the job of sizing centrifugal pumps. Since most applications in drilling systems require 80 feet of head at the inlet of the equipment, knowledge of volume needed by each piece of equipment is required. Following are standard flow rates when equipment has an 80-foot inlet head:

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TURBULENT FLOW OF CENTRIFUGAL PUMP

Turbulent flow is detrimental to a centrifugal pump during handling of abrasive fluids. The drilling industry has standardized centrifugal pumps with concentric casings and wide impellers, a design that has proven to offer less turbulence and greatest pump life. The walls of a concentric style of casing (Figure 1) are an equal distance from the impeller throughout the impeller circumference, resulting in a smooth flow pattern. A volute style of casing (Figure 2) has a cutwater point that disturbs the fluid flow pattern, creating an eddy.

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