Marine Engineering Study Materials

Starting and Running of Screw Pumps

• The complete pipe line system must be flushed and pressure tested before installing and operating the pump. If any corrosive liquid is used for the same, then complete system to be properly drained and dried for the protection of the complete system. 
• Before starting the pump for the first time, it must be completely filled with working liquid as shown in figure below.

• Line up all the valves in the pumping system to avoid liquid pressure build up as well as pump losing suction.
• Make sure the prime mover motor is isolated electrically and turn the pump-motor coupling by hand to check it is turning smoothly.
• When pump is ready to be started, first briefly switch on and off the motor and check the direction of rotation is as per the marking on the pump.
• Start the pump and keep checking suction and discharge pressure gauges for rated pressure.
• Check prime mover motor ampere and compare with rated current.
• Check for any leakages from mechanical seal or other joints of the pump, flanges, etc.
• Air in the pumping liquid causes abnormal vibration, noise and over heating of the pump and liquid which may result in a fire hazard.
• Periodically inspect foundation bolts, coupling pads, leakages and performance of the pump as condition monitoring to avoid breakdown maintenance.

Use of Pressure Relief Valve in Pump

• All systems with screw pumps must be equipped with a pressure relief valve installed immediately adjacent to the pump.
• In normal medium capacity pumps, this pressure relief valve is an integral part of the pump to protect the system against excess pressure.
• When liquid is circulated through the valve it heats up in proportion to the set pressure level and the percentage of by-passed liquid.
• 100% bypass can only be tolerated for less than about 3 minutes.
• 50 %by-pass generally for unlimited periods of time.
• If more than 50% recirculation is anticipated, a value specific to each application should be determined by closely monitoring the pump body temperature.

Maintenance of Screw Pumps

• Pump to be overhauled at regular intervals, not exceeding 3 years.
• Wear of spare parts greatly depends on the pumping medium. Pump screws, liner, etc. are lubricated by the pumping liquid itself. So it is important to avoid presence of any abrasive particles in the pumping medium. Hence pump strainer must be cleaned regularly, by monitoring suction pressure of the pump. A gradual reduction in the suction pressure of the pump indicates that the suction filter is getting chocked.
• Indication of pump parts wear can be identified from abnormal noise, vibration, loss of capacity, reduction in discharge pressure, etc.
• Inspect internal parts carefully while overhauling. Internal clearances in the pump, which are vital for its proper function, may have been affected by wear of rotors and bores or liner. Acceptable wear can be determined only by experience of the actual application. As a rule of thumb the following max clearance values may apply: Between rotor and bores: 0.2 mm, Between rotor flanks: 0.4 mm.
• Inspect for any scratches inside.
• Check the condition of the mechanical seal especially the mating faces and o-rings. Discard the seal if mating face is damaged or o-rings hardened. Excessively leaking shaft seals (more than 10 drops per hour) should be changed without delay, as the leakage normally will grow worse and cause additional damage.
• Gaskets and o-rings of the pump to be renewed while overhauling.
• Check shaft bearing for damages and renew if necessary.
• Relief valve, valve seat, springs to checked.
• It is advisable to overhaul the motor also along with the pump.

Troubleshooting of Screw Pumps

Problem	Cause	Remedy
Wrong direction of rotation	Electric cables to motor wrongly connected.	Reverse the terminal connection on electric motor.
The pump cannot be primed	Wrong direction of rotation. Suction line is not open or pressure drop in the suction line is too high. Major air leakage into the suction line. The pump cannot evacuate the air through the discharge line due to excessive counter pressure.	See above. Check all components in suction line. The inlet condition should be checked with a vacuum gauge at the pump inlet. Check the suction line. Discharge the air through air purge cock at the discharge of the pump.
No flow	The pump is not primed. The pressure relief valve is set below the counter pressure.	See above. Readjust the pressure relief valve to a value above counter pressure.
Flow too low	The coupling is slipping The pressure relief valve is set too low (Discharge pressure also low). Something is restricting the flow in the suction line. (This would usually cause noise). The pumped liquid contains a significant amount of compressible gas, such as free air. (This would usually cause noise).	Check the condition of the coupling. Re-tighten, renew coupling pads. Readjust the pressure relief valve to get rated discharge pressure. Check all components in the suction line (strainers, valves etc.). Go through the system and determine if there are any leaks. Rectify if any.
Pressure too low	The pressure relief valve is set too low. Counter pressure in the discharge line is too low due to a major leakage. The valve piston is stuck in open position. Something is restricting the flow in the suction line. (This would usually cause noise). The pumped liquid contains a significant amount of compressible gas, such as free air. (This would usually cause noise). A too small pump has been chosen.	Readjust the pressure relief valve. Check the components in the discharge line inclusive the recipients. Check the valve. Check all components in the suction line (strainers, valves etc.). Go through the system and determine if there are any leaks. Rectify if any. Check the capacity of the pump.
Pressure too high	The pressure relief valve is set too high. The oil is too cold (or has higher viscosity than anticipated). Counter pressure in the discharge line is too high.	Readjust the pressure relief valve. Reduce the pressure setting until operational temperature has been reached. Check the discharge line.
Drive motor difficult to start or tends to stop by tripping the motor overload relay	Counter pressure too high. Liquid too cold. Motor is undersized for the prevailing conditions. Electrical power supply faulty. Motor overload relay set too low or is faulty. Incorrect setting of Y/D starter.	See above: Pressure too high. Readjust the pressure relief valve to a lower value. Thus the power consumption for the pumping is relieved and overloading due to the high viscosity may be avoided. When the liquid has reached normal temperature and thus flows easily, the relief valve is reset to normal pressure. Check the motor. Check the motor and motor connection. Readjust or replace the relay. Readjust the setting of the starting sequence. The time before the motor overload relay is tripped should not exceed 10-15 seconds.
Noise and vibration	The flow to the pump is insufficient. Insufficient support of pipe work. Air leakage into the suction line. Faulty electrical supply.	See chapter: The flow is too low. Check for pipe vibrations in the pump connections. Check that the pipes are sufficiently clamped. Check the suction line for air leakage. Check all three phases of the supply.

Reference: IMO ACE SCREW PUMPS