Q. If industry guidelines for intake design are followed, when is it advisable to conduct physical model tests of intake structures?
A. Industry guidelines cannot always be followed. A properly conducted physical model study is a reliable method to identify unacceptable flow patterns at the pump suction for given sump or piping designs and to derive acceptable intake sump or piping designs. Considering the cost for a model study, an evaluation is needed to determine if a model study is required. A physical hydraulic model study shall be conducted for pump intakes with one or more of the following features:
- Sump or piping geometry (bay width, bell clearances, side wall angles, bottom slopes, distance from obstructions, the bell diameter or piping changes, etc.) that deviates from this design standard.
- Non-uniform or non-symmetric approach flow to the pump sump exists (e.g., intake from a significant cross-flow, use of dual flow or drum screens or a short radius pipe bend near the pump suction, etc.).
- The pumps have flows greater than 9,100 m3/h (40,000 gpm) per pump or the total station flow with all pumps running would be greater than 22,700 m3/h (100,000 gpm).
- The pumps of an open bottom barrel or riser arrangement have flows greater than 1,140 m3/h (5,000 gpm) per pump.
- Proper pump operation is critical and pump repair, remediation of a poor design and the impacts of inadequate performance or pump failure all together would cost more than 10 times the cost of a model study.
When evaluating the indirect impacts of inadequate performance or pump failures, the probability of failure may be considered, such as by comparing the proposed intake design to other intakes of essentially identical design and approach flow that operate successfully. The model study shall be conducted by a hydraulic laboratory using personnel with experience in modeling pump intakes.
Adverse hydraulic conditions that can affect pump performance include: free and sub-surface vortices, swirl approaching the pump impeller, flow separation at the pump bell and a non-uniform axial velocity distribution at the suction.
Free-surface vortices are detrimental when their core is strong enough to cause a (localized) low pressure at the impeller and because a vortex core implies a rotating rather than a radial flow pattern. Sub-surface vortices also have low core pressures and are closer to the impeller. Strong vortex cores may induce fluctuating forces on the impeller and cavitation. Sub-surface vortices with a dry-pit suction inlet are not of concern if the vortex core and the associated swirling flow dissipate well before reaching the pump suction flange.
Pre-swirl in the flow entering the pump exists if a tangential component of velocity is present in addition to the axial component. Swirl alters the inlet velocity vector at the impeller vanes, resulting in undesired changes in pump performance characteristics, including potential vibration.
A reasonably uniform axial velocity distribution in the suction flow (approaching the impeller) is assumed in the pump design, and non-uniformity of the axial velocity may cause uneven loading of the impeller and bearings.
A properly conducted physical model study can be used to derive remedial measures, if necessary, to alleviate these undesirable flow conditions due to the approach upstream from the pump impeller. The typical hydraulic model study is not intended to investigate flow patterns induced by the pump itself or the flow patterns within the pump. The objective of a model study is to ensure that the final sump or piping design generates favorable flow conditions at the pump inlet.
Models involving a free surface are operated using Froude similarity since gravity and inertial forces control the flow process. Details on this can be found in HI Standard ANSI/HI 9.8, Pump Intake Design.
Q. Reciprocating power pumps are usually operated at speeds limited by the manufacturer. What are the factors that contribute to this limit?
A. Factors affecting pump maximum operating speed include the following:
- Liquid characteristics: Temperature, viscosity, corrosiveness, compressibility, the presence of solids and the presence of dissolved or entrained gas
- Application details: NPSH available, piping design and layout, pulsation dampeners (if any), the ambient temperature, shelter, foundation, driving machinery, protective shut-down devices used, the accessibility of factory service personnel, spare parts and overhaul facilities, as desired
- Pump design: Including valve material, size and type, piston, diaphragm or plunger construction, the choice of packing and packing lubrication, if any, materials used in liquid end and trim, the method of driving pump and NPSHR
- Type of duty:
- Continuous duty: Eight to 24 hours per day, fully loaded
- Light duty: Three to eight hours per day, fully loaded
- Intermittent duty: Up to three hours per day, fully loaded
- Cyclical operation: ½ minute loaded out of every three minutes
- Maintenance level: Attended or unattended operation. Skill, training and tools of operating and maintenance personnel.
Power pump speeds at or near the manufacturer's published "rated" or "normal" curve include those applications when clean, cold liquids are involved and provide long life and economical operation, if all important application details are carefully handled and regular, skilled maintenance is provided.
Medium speed selection requires excellent piping layout, good environment, adequate NPSHA, periodic preventive maintenance and lubrication, rigidly fixed piping, and solid pump and prime mover foundations or bases. It may require automatic safety shutdown devices, suction and discharge dampeners and plunger or piston rod packing lubrication.
Medium speeds may be too fast for slurries, marginal NPSH situations or unattended operation.
Slow Speed Applications
Selection of an operating speed below the manufacturer's "rated" or "normal" speed curve is often desirable when any strongly adverse factor is present, such as the following:
- Abrasive liquid (slurry)
- Hazardous liquid
- Extreme pressure
- Corrosive chemical
- High viscosity
- Unattended operation
- Poor maintenance
- No spare parts, or no standby pump
- High liquid temperature
- High ambient temperature
- Extremely long life desired
- High-cost downtime of related facilities
- Extreme isolation of site
- Radioactive liquid
- Dissolved gas in liquid
- Borderline suction (NPSHA) situation
Operation at extremely slow speeds may require supplementary power end lubrication. Cooling of the power end oil may be necessary when hot liquids or ambients occur. Always consult the manufacturer when very hot or very cold liquids are involved. Revisions may be required in construction for these application types.
High Speed Applications
Selection of speed above manufacturer's "rated" or "normal" curve and/or near his "maximum" or "intermittent" curve (if any) is sometimes merited when intermittent, attended service is involved. High speed selection requires close attention to all application details, skilled operators and proper pump design. A suction booster pump may be required to obtain sufficient NPSHA.
High speeds imply that only optimum application factors are present and reduced life may occur. Some pumps are inherently designed for high-speed, short duration and infrequent usage. All conditions of such service should be well understood by all parties prior to the sale. Oil well fracturing, acidizing and cemented plunger pumps are examples of this type of high speed, intermittent application.
Pumps & Systems, May 2010
For additional information regarding reciprocating pumps, see ANSI/HI 6.1-6.5, Reciprocating Power Pumps for Nomenclature, Definitions, Application, and Operation.