433A Pumps and Compressors 4 — Questions and Answers
Question 1: What is a progressive cavity pump and what fluids is it best suited for?
- A pump used in clean water applications only
- A positive displacement pump using a helical rotor in a rubber stator, ideal for viscous, shear-sensitive, and solids-laden fluids (Correct answer)
- A high-speed centrifugal pump
- A pump that progressively increases in size
Correct answer: A positive displacement pump using a helical rotor in a rubber stator, ideal for viscous, shear-sensitive, and solids-laden fluids
A progressive cavity pump uses a single helical rotor turning inside a double helical rubber stator, creating progressing sealed cavities that gently move fluid without pulsation. It handles viscous, abrasive, and shear-sensitive fluids including slurries and sludge.
Question 2: What is deadheading a centrifugal pump and why is it dangerous?
- Running the pump with the suction valve closed
- Operating the pump with the discharge valve closed, causing the pump to churn fluid internally and overheat rapidly (Correct answer)
- Running the pump at maximum speed
- Operating the pump without a motor
Correct answer: Operating the pump with the discharge valve closed, causing the pump to churn fluid internally and overheat rapidly
Deadheading occurs when the pump runs with the discharge closed (or severely restricted). All the input energy converts to heat in the confined fluid, causing rapid temperature rise that can vaporize the fluid, damage seals and bearings, and potentially rupture the casing.
Question 3: What type of compressor uses centrifugal force to compress gas?
- Reciprocating compressor
- Centrifugal (dynamic) compressor (Correct answer)
- Scroll compressor
- Diaphragm compressor
Correct answer: Centrifugal (dynamic) compressor
A centrifugal compressor uses a high-speed rotating impeller to accelerate gas outward by centrifugal force, then converts this velocity energy to pressure in a diffuser and volute. It is used for large-volume, continuous-flow applications.
Question 4: What is the purpose of a shaft sleeve on a pump?
- To increase the shaft diameter
- To protect the shaft from wear and corrosion at the packing or seal area, providing a replaceable wear surface (Correct answer)
- To transmit torque to the impeller
- To support the pump bearings
Correct answer: To protect the shaft from wear and corrosion at the packing or seal area, providing a replaceable wear surface
A shaft sleeve is a replaceable protective cover fitted over the shaft in the area where packing or seals contact it. It protects the more expensive shaft from the wear, corrosion, and erosion that occurs at the sealing area.
Question 5: What is surge in a centrifugal compressor?
- A sudden increase in speed
- An unstable operating condition where flow reverses direction periodically, causing violent pulsation and potential mechanical damage (Correct answer)
- A normal startup condition
- Excess lubrication in the compressor
Correct answer: An unstable operating condition where flow reverses direction periodically, causing violent pulsation and potential mechanical damage
Surge occurs when flow through a centrifugal compressor drops below a minimum value, causing a momentary flow reversal. The cycle of forward flow, stall, reversal, and recovery creates violent pressure pulsations and can cause catastrophic mechanical damage.
Question 6: When installing a new pump impeller, why is impeller clearance critical?
- Clearance does not affect pump performance
- Proper clearance minimizes internal recirculation losses while preventing rubbing contact between the impeller and casing (Correct answer)
- Clearance only affects noise levels
- Maximum clearance always gives the best performance
Correct answer: Proper clearance minimizes internal recirculation losses while preventing rubbing contact between the impeller and casing
Impeller clearance must be set precisely per manufacturer specifications. Too tight and the impeller may rub the casing, causing damage. Too loose and excessive internal recirculation reduces efficiency, head, and flow output.
433A Pumps and Compressors 4
What is a progressive cavity pump and what fluids is it best suited for?