Pumps intended for heavy-duty services (like refinery or petrochemical) should typically comply with API-610. Lighter-duty process pumps may be specified to other standards, such as ANSI. Sealing systems for API-610 pumps generally follow API-682.
Pump Classifications
Impeller Type
Closed: Vanes are enclosed on both sides. This type has the highest hydraulic efficiency and is specified for all clean liquid duties. It should not be used for liquids with solids that could clog the passages.
Semi-Open: Vanes are visible from one side. This design offers good slurry handling characteristics with a hydraulic efficiency between closed and open types.
Open: Vanes are fully visible. This type has the lowest efficiency but is sometimes used for services with stringy or pulpy material.
Priming
Non-priming pumps: This is the most common type. They cannot remove gases or vapors from the suction line and require the casing and suction pipe to be completely filled with liquid before starting (known as "flooded suction").
Self-priming pumps: These are designed with a priming chamber or air separator that allows them to displace vapors from the inlet pipe. They are useful for suction lift conditions but have lower efficiency than non-priming pumps.
Design and Performance
Design Margins
The "rated capacity" of a pump is determined by applying a design margin to the "normal capacity" (the mass balance flow rate). Typical margins are:
Pump Service
Typical Capacity Margin
Recirculation Pump
0%
Large Cooling Water Pump
5%
Transfer Pump
5%
Process Pump
10%
Reflux Pump
20%
Boiler Feed Water (BFW) Pump
20%
Head Margin: A 10% design margin is typically added to the calculated frictional loss component of the total head.
NPSH Margin: A margin of 10% or 0.6 m (whichever is higher) should be added to the Net Positive Suction Head Available (NPSHa).
Pump Characteristic Curves
The performance of a pump is shown on its characteristic curve, which plots the Head (ΞH) it produces versus the Flow Rate (Q). A pump will always operate at the intersection of its pump curve and the "system curve."
Pump Curve: Represents the head the pump can deliver at a given flow. Curves can be:
Rising: Head continuously rises as flow decreases to zero (shutoff). This is a "stable" curve.
Falling: Head reaches a maximum at some flow rate, then drops as flow decreases to shutoff. This is an "unstable" curve.
System Curve: Represents the total resistance of the piping system. It is the sum of:
Static Head: The physical elevation change.
Pressure Head: The difference in pressure between the destination and source vessels.
Frictional Loss: Head lost due to friction in pipes, valves, and fittings (this varies with the square of the flow rate).
The Best Efficiency Point (BEP) is the point on the curve where the pump operates at its maximum efficiency. The selected operating point should be close to the BEP, preferably slightly to the left (lower flow). Operating far to the right of the BEP can lead to a sharp drop in head and a sharp rise in required NPSH.
Net Positive Suction Head (NPSH)
NPSH is the total head at the pump inlet (centerline) minus the liquid's vapor pressure. It ensures the liquid does not vaporize inside the pump.
NPSHa (Available): The absolute head that exists in the system at the pump suction. This is a characteristic of your system design.
NPSHR (Required): The minimum head required by the pump to prevent vapor formation and cavitation. This is a characteristic of the pump.
Rule: You must always ensure NPSHa > NPSHR (including the design margin). If NPSHa is too low, the liquid will flash, and the resulting bubbles will collapse, causing "cavitation" β a phenomenon that creates high noise, vibration, and severe damage to the impeller.
Suction Lift
This condition occurs when the pump is located above the liquid source. The pump creates a partial vacuum, and atmospheric pressure on the liquid surface pushes the fluid up. The practical maximum suction lift for water is around 5-6 meters. Suction lift capability is reduced by:
Higher Altitude: Lower atmospheric pressure.
Higher Temperature: Higher liquid vapor pressure.
Higher Density: A denser fluid is heavier to lift.
Maximum Casing Pressures
Max Suction Pressure: Calculated based on the suction system's relieving pressure plus the maximum static liquid head.
Max Discharge Pressure: Calculated as the Max Suction Pressure plus the pump's differential pressure at shutoff (zero flow). (Shutoff head can be estimated as 120% of the operating head if the curve is unavailable).
Operational Considerations
Minimum Flow Protection
All centrifugal pumps require a minimum flow to operate safely. Operating below this flow can cause significant problems.
Reasons for Minimum Flow
Overheating: At low flow, most of the driver's power is converted to heat, which can vaporize the liquid inside the pump.
Mechanical Instability: Low flow can cause excessive vibration, which can damage seals and bearings.
A minimum flow bypass is required if the process flow can drop below the pump's minimum, such as when a discharge control valve throttles closed.
Bypass Control Methods
Continuous Bypass: A simple orifice recirculates a fixed flow back to the suction tank. This is simple but wastes energy and is typically only for small pumps (e.g., < 10 mΒ³/h).
Automatic Bypass: An instrumented loop with a flow meter and control valve that only opens when the process flow drops to the minimum. Automatic recirculation valves (ARVs) that combine check, bypass, and sensing functions are also available.
Variable Speed Drive (VSD): Can be used to extend the operating range, but the pump still has a minimum speed limit for cooling and stability.
Bypass Philosophy
Individual Bypass: Each pump (including the standby) has its own bypass. This is the safest option.
Common Bypass: One bypass line in the common discharge header. This is cheaper but does not protect a pump if its individual discharge valve is accidentally closed while running. This is only suitable for identical pumps.
Troubleshooting Inadequate NPSH
If your calculated NPSHa is less than the pump's NPSHR, you must modify the system or the pump. You have two options:
How to Decrease NPSHR (Required)
Use a pump with a lower speed (RPM).
Select a pump with a larger inlet nozzle (eye) size.
Use a double-suction pump (which splits the flow in half to each side).
Add an "inducer," a small axial impeller, ahead of the main impeller.
How to Increase NPSHa (Available)
Raise the liquid level in the suction tank (e.g., raise the tank or increase the low-level setpoint).
Lower the pump elevation.
Reduce friction in the suction line (e.g., use a larger pipe, use long-radius elbows, shorten the pipe run).
Cool the liquid to reduce its vapor pressure (but ensure the cooler itself doesn't add too much friction).
Increase the pressure in the suction tank (only if the gas won't dissolve and the liquid is not already at its bubble point).
Use a low-NPSHR "booster pump" to feed the main pump.
Pump Sealing Systems
Seal Selection
Gland Packing: Used for non-hazardous services like cooling water.
Single Mechanical Seal: The standard for most process pumps.
Dual Mechanical Seals: Specified for hazardous services:
C4 and lighter liquids (e.g., LPG)
Liquids with high vapor pressure
Toxic, sour, or environmentally hazardous liquids
Liquids above their auto-ignition temperature
API Seal Plan Philosophies
API plans provide a standard way to manage the environment around the seal. The general philosophies are:
Plans 1-13: For single seals with cool, clean fluids (simple recirculation).
Plans 21-23: For single seals with hot, clean fluids (uses a cooler).
Plans 31-41: For single seals with dirty fluids (uses a cyclone separator or filter).
Plan 52: For dual seals (unpressurized "buffer" fluid between seals).
Plans 53 & 54: For dual seals (pressurized "barrier" fluid from an external source, at a pressure > process).
Plan 62: An external "quench" (like low-pressure steam) to prevent buildup or dilute leakage.
Dual seals require a "barrier" or "buffer" fluid. This fluid must be clean and compatible with the pumped liquid.
Seal-less Pumps
For applications requiring zero leakage (e.g., toxic, explosive, or very costly liquids), seal-less pumps are an option. They have limitations and should only be used if the fluid is clean (no solids/crystals), low viscosity (< 40 cP), and has a large margin to its boiling point.
Magnetic Drive (Mag-Drive): Power is transferred via a magnetic coupling.
Canned Motor: The rotor/impeller is a single assembly, and the motor stator is "canned" off from the process fluid.
Parallel and Series Operation
Parallel Operation
Two or more pumps discharge into a common header. This is used for redundancy, flexibility, or when one large pump is not feasible. The combined curve is found by adding the flow rates at the same head.
Key Considerations:
Stable Curve: Pumps must have a stable (continuously rising) curve. Unstable pumps operating in parallel can "hunt," causing vibration and unstable output.
Flow is Not Doubled: Running two identical pumps will not double the flow. As flow increases, system friction increases, and the operating point moves up the system curve. The total flow will be less than 2x the single-pump flow.
Trip Scenario: If one of two running pumps trips, the remaining pump will "run out" on its curve to a much higher flow rate. You must check that the motor will not be overloaded and that NPSHa is still sufficient at this new, higher flow.
Series Operation
One pump discharges into the suction of another. This is used to achieve a very high head (like a booster pump). The combined curve is found by adding the heads at the same flow rate.
Key Considerations:
Trip Scenario: If one pump trips, the total head drops significantly, causing the flow to drop to a new, low operating point. You must check that this new flow rate is not below the pump's minimum flow requirement.
Modifications and Special Cases
Debottlenecking and Affinity Laws
The performance of a pump can be altered by changing its speed or impeller diameter. These changes are governed by the Affinity Laws (which are a good approximation for small changes, e.g., < 15%).
Affinity Laws
Capacity (Flow): Varies directly with speed (n) or diameter (d).
(Q β n or Q β d)
Head: Varies with the square of the change in speed or diameter.
(H β nΒ² or H β dΒ²)
Power: Varies with the cube of the change in speed or diameter.
(P β nΒ³ or P β dΒ³)
System Modifications
When adding a new pump in parallel to debottleneck, remember that flow will not double. Simultaneous efforts to reduce the system head (e.g., increasing line size or reducing control valve pressure drop) should also be investigated. When replacing a pump, if the liquid density is different, you must check the new power requirement to ensure the motor is not overloaded.
Special Pumping Requirements
Slurry Pumping: Requires special materials for erosion, attention to non-Newtonian flow characteristics, and maintaining minimum velocities to prevent solids from settling.
Viscous Liquids: Centrifugal pumps are generally not recommended for viscosities above 300 cSt. Pumping liquids > 10 cSt will result in lower head, lower efficiency, and increased NPSHR compared to water performance.
End-of-Curve Operation: Pump motors are normally sized for the operating point. However, power draw is highest at the "end of the curve" (open discharge). If a pump has an "auto-start" function where manual throttling is not possible, the motor must be sized for this end-of-curve power to prevent a burnout.
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