Pump suction specific speed calculation and limitations

Suction Specific Speed (NSS)

Suction Specific Speed is another dimensionless parameter used in centrifugal pumps. Much of the discussion on specific speed (NS)applies to suction specific speed (NSS).

While specific speed (NS) is mostly related to the discharge side of the pump, the suction specific speed deals primarily with its suction (inlet) side. The head (H) term in the denominator of the defining formula for the NS is substituted by the NPSHR:

NSS = RPM x Q^0.5 / NPSHR^0.75, where

flow is in gpm, and NPSHR is in feet.

Values of NSS vary from about 6,000 to 15,000, and sometimes even higher for the specialized designs.

From the discussion of a pump suction performance (see topic "How does pump suction limit the flow?"), we know that conflicting demands are imposed on a pump system by the pump user and a pump manufacturer.

A user would prefer to provide as low NPSHA as possible, as it often relates to a system cost: for example, higher level of liquid in the basin of the cooling water pumps requires taller basin walls, or deeper excavation to lower a pump centerline below the liquid level. A pump manufacturer, on the other hand, wants to have more NPSHA, with an ample margin above the pump NPSHR, to avoid cavitation, damage, and similar problems.

In other words, a wider margin (M) can be achieved either by increasing the NPSHA, or decreasing the NPSHR, since

M = NPSHA - NPSHR

Thus it may appear that a lower NPSHR design is preferential, and a competing pump manufacturer might present a lower NPSHR design as automatically translated into construction cost savings - because of not having to increase the NPSHA. Since a lower NPSHR design means a higher value of NSS (according to the definition above), the highest NSS design might seem to look best. In reality, however, this is not so.

In a topic "How does pump suction limit the flow?" it was explained that higher flow velocities result in reduction of the static pressure, which may then become dangerously close to the fluid vapor pressure and cavitation. Thus, lower velocities result in higher localized static pressure, i.e. a safer margin from the cavitating (i.e. vaporization) regime. Since the velocity is equal to flow divided by the area, a larger area (for the given flow) reduces the velocity, - a desirable trend.

This is why a larger suction pipe is beneficial at the pump inlet. Cavitation usually occurs in the eye region of the impeller, and if the eye area is increased - velocities are decreased, and the resulting higher static pressure provides a better safeguard against vaporization (cavitation). So, a larger impeller eye seems like a way to lower the NPSHR:

Figure 3-1 Larger impeller eye results in lower NPSHR at BEP, but certain problems arise at off-peak operation

Unfortunately, the flow of liquid at the impeller eye region is not as simple and uniform as it is in a straight run of a suction pipe. Impeller eye has a curvature, which guides the turning fluid, like a car along the sharp curves of the road, into the blades and towards the discharge. If a pump operates very close to its BEP, the inlet velocity profile becomes proportionally smaller, but the fluid particles stay within the same paths:

If, however, a pump operates below its BEP, the velocity profile changes, and no longer can maintained its uniformity and order. Fluid particles then begin to separate from the path of the sharpest curvature (which is the impeller shroud area), and the resulting mixing and wakes produce a turbulent, disorderly flow regime, which makes matters difficult from the NPSHR standpoint. 

Figure 3-2: Even thought large eye impeller has better NPSHR at BEP, it has flow separation problems at low flow

The upshot of all this is that a larger impeller eye does decrease the NPSHR at the BEP point, but causes flow separation problems at the off-peak low-flow conditions. In other words, a high Suction Specific Speed (NSS) design is better only if a pump does not operate significantly below its BEP point.

Interestingly, with very few exceptions, there is hardly a case where a centrifugal pump operates strictly at the BEP. The flow demands at the plants change constantly, and operators throttle the pump flow via the discharge valve. High NSS designs are known to result in reliability problems because of such frequent operation in the undesirable low flow region. Actual plant studies have shown, that above NSS of 8500 - 9000, pumps reliability begins to suffer - exponentially:

Figure 3-3 Plant experience shows that impellers designed with NSS greater than 9000 have poor reliability record

Realizing this, around mid-80s, users started to limit the value of the NSS, and a Hydraulic Institute uses NSS = 8500 as a typical guiding value. It might be of interest for you to calculate the values of NSS of your pumps, and find out if a correlation between those and reliability exists at your plant.

Limits of bearing element spead,mean diameter and rated power density

Accoring to operation condition bearing selection and arrangement

The use of threaded holes in pressure parts in centrifugal pumps pumps

The use of threaded holes in pressure parts shall be minimised. To prevent leakage in pressure sections of casings, metal, equal in thickness to at least half the nominal bolt or stud diameter, plus the allowance for corrosion, shall be left around and below the bottom of drilled and threaded holes.

a) Internal bolting shall be of a material fully resistant to corrosive attack by the pumped liquid.

b) Studs shall be supplied on all main casing joints unless cap screws are specifically approved by the purchaser.

O-ring sealing surfaces(maximum surface roughness)(Bores)

O-ring sealing surfaces, including all gr ooves and bores, shall have a maximum surface roughness
average value (R a) of 1,6 mm (63 micro inches) for static O-rings and 0,8 mm (32 micro inches) for the surface against which dynamic O-rings slide.

Bores shall have a minimum 3mm (0,12 in) radius or a minimum 1,5 mm (0,06 in) chamfered lead-in for static O-rings and a minimum 2 mm (0,08 in) chamfered lead -in for dynamic O-rings.

Chamfers shall have a maximum angle of 30°.

Pumps with Radially split casings and Axially-split casings operating temperature and pressure ranges

Unless otherwise specified, pumps with radially split casings are required for any of the following operating conditions.

a) A pumping temperature of 200 °C (400 °F) or higher (a lower temperature limit should be considered if thermal shock is probable).

b) A flammable or hazardous pumped liquid with a relative density of less than 0,7 at the specified pumping temperature.

c) A flammable or hazardous pumped liquid at a rated discharge pressure above 10 000 kPa (100 bar)

(1 450 psi). Axially-split casings have been used successfully beyond the limits given above, generally for off-plot applications at higher pressure or lower relative density (specific gravity). The success of such applications depends on the margin between design pressure and rated pressure, the manufacturer’s experience with similar applications, the design and manufacture of the split joint, and the user’s ability to correctly remake the split joint in the field. The purchaser should take these factors into account before specifying an axially-split casing for conditions beyond the above limits.

Pumps with heads greater than 200 m (650 ft) per stage and with more than 225 kW (300 hp) per stage Percent clearance calculation

Pumps with heads greater than 200 m (650 ft) per stage and with more than 225 kW (300 hp) per stage
may require special provisions to reduce vane passing frequency vibration and low -frequency vibration at reduced flowrates. 

For these pumps, the radial clearance between the diffuser vane or volute tongue (cutwater) and the periphery of the impeller blade shall be at least 3 % of the maximum impeller blade tip radius for diffuser designs and at least 6% of the maximum blade tip radius for volute designs. The maximum impeller blade tip radius is the radius of the largest impeller that can be used within the pump casing . 

Percent clearance is calculated as follows:

  P = 100 (R 3- R2)/ R2

where
P is the percent clearance
R3 is the radius of volute or diffuser inlet tip
R2 is the maximum impeller blade tip radius.

The impellers of pumps covered by this clause shall not be modified after test to correct hydraulic performance byunderfiling, overfiling, or “V” cutting without notifying the purchaser prior to shipment.

Pumps with heads greater than 200 m (650 ft) per stage and with more than 225 kW (300 hp) per stage

Pumps with heads greater than 200 m (650 ft) per stage and with more than 225 kW (300 hp) per stage
may require special provisions to reduce vane passing frequency vibration and low -frequency vibration at reduced flowrates. 

For these pumps, the radial clearance between the diffuser vane or volute tongue (cutwater) and the periphery of the impeller blade shall be at least 3 % of the maximum impeller blade tip radius for diffuser designs and at least 6% of the maximum blade tip radius for volute designs. The maximum impeller blade tip radius is the radius of the largest impeller that can be used within the pump casing . 

Percent clearance is calculated as follows:

  P = 100 (R 3- R2)/ R2

where
P is the percent clearance
R3 is the radius of volute or diffuser inlet tip
R2 is the maximum impeller blade tip radius.

The impellers of pumps covered by this clause shall not be modified after test to correct hydraulic performance byunderfiling, overfiling, or “V” cutting without notifying the purchaser prior to shipment.

The maximum continuous operating speed of centrifugal pump

Pumps shall be capable of operating at least up to the maximum continuous speed. The maximum continuous speed shall be:

a) equal to the speed corresponding to the synchronous speed at maximum supply frequency for electrical motors.

b) at least 105 % of rated speed for variable-speed pumps, and any fixed speed pump sparing or spared by a pump whose driver is capable of exceeding rated speed.

Spiral plate Heat exchanger operation temperature and pressure ranges

Spiral plate Heat exchanger 

Of all the compact exchangers, the spiral is certainly the most unique. For example, all of the above-mentioned types are based on thin-plate technology, with multiple flow paths created by either corrugation of the plates or the use of a fin structure between the plates. They are also designed for very high efficiencies.

A spiral exchanger is a configuration that, in cross section, resembles a watch spring, using two heavy-gage strips of material to create one flow channel for each fluid. It was developed to handle problem fluids, such as those that have severe fouling tendencies or contain solids in suspension.

A novel feature of the exchanger, when laid on its side, is its hydraulics. Rapid changes in cross-sectional geometry perpendicular to flow counteract the settling of suspended particles. The constantly changing direction of the wall also creates higher turbulence than in shell-and-tube units, and limits the amount and rate of scale deposition. In normal configurations, both fluid channels, while welded off from each other, can be easily accessed for cleaning by removal of the heads, without removal of the internals.

Because flow is countercurrent, the spiral can handle very deep temperature crosses and achieve closer approaches than can shell-and-tube exchangers. Service that would normally require several shells stacked in series, can generally be handled in a single spiral. Typical duties include feed-effluent exchangers, slurry exchangers on coke- or catalyst-containing streams, minerals or fibers, and applications where space is limited.

With modification of its internal design, the spiral can also be used as a condenser. In this service, maximum benefit can be obtained if the unit is mounted atop the column. This arrangement is easy to do because of the small size, and allows for running the reflux back by gravity alone. Also, this setup eliminates the overhead and reflux lines, the drum and pump, and the foundations and space required for a grade-mounted unit. The elimination of the ancillary equipment can potentially return a savings several times the cost of the condenser.

Limitations of the spiral are temperature and pressure, with a maximum pressure of 25 bars and 500 degrees Celsius as the top temperature limit. These limits vary, however, depending on the unit's size and material of construction. Volumetric flowrate may also limit use of the spiral exchanger. Because of its single channel, maximum flow is about 350 m³/h, which is generally less than that of shell-and-tube models.

Spiral plate exchanger advantages and disadvantages

Spiral plate Heat exchanger 

Of all the compact exchangers, the spiral is certainly the most unique. For example, all of the above-mentioned types are based on thin-plate technology, with multiple flow paths created by either corrugation of the plates or the use of a fin structure between the plates. They are also designed for very high efficiencies.

A spiral exchanger is a configuration that, in cross section, resembles a watch spring, using two heavy-gage strips of material to create one flow channel for each fluid. It was developed to handle problem fluids, such as those that have severe fouling tendencies or contain solids in suspension.

A novel feature of the exchanger, when laid on its side, is its hydraulics. Rapid changes in cross-sectional geometry perpendicular to flow counteract the settling of suspended particles. The constantly changing direction of the wall also creates higher turbulence than in shell-and-tube units, and limits the amount and rate of scale deposition. In normal configurations, both fluid channels, while welded off from each other, can be easily accessed for cleaning by removal of the heads, without removal of the internals.

Because flow is countercurrent, the spiral can handle very deep temperature crosses and achieve closer approaches than can shell-and-tube exchangers. Service that would normally require several shells stacked in series, can generally be handled in a single spiral. Typical duties include feed-effluent exchangers, slurry exchangers on coke- or catalyst-containing streams, minerals or fibers, and applications where space is limited.

With modification of its internal design, the spiral can also be used as a condenser. In this service, maximum benefit can be obtained if the unit is mounted atop the column. This arrangement is easy to do because of the small size, and allows for running the reflux back by gravity alone. Also, this setup eliminates the overhead and reflux lines, the drum and pump, and the foundations and space required for a grade-mounted unit. The elimination of the ancillary equipment can potentially return a savings several times the cost of the condenser.

Limitations of the spiral are temperature and pressure, with a maximum pressure of 25 bars and 500 degrees Celsius as the top temperature limit. These limits vary, however, depending on the unit's size and material of construction. Volumetric flowrate may also limit use of the spiral exchanger. Because of its single channel, maximum flow is about 350 m³/h, which is generally less than that of shell-and-tube models.

Diffusion-bonded Heat exchangers ,Plate-fin model,Printed-circuit model Advantages and disaadvantages

Diffusion-bonded Heat exchangers 

Technological innovations continue to extend the range of compact heat exchangers, pushing the temperature threshold to 1,000 degrees Celsius and pressure to 500 bars, depending on the material of construction and type of exchanger. Diffusion bonding creates a block of ``parent metal'' with uniform grain structure throughout the heat exchanger, even across what were once individual plates. The result is a unit with no joints or seams other than the external connection attachments.

Plate-fin model. Produced by a joint venture of Rolls Royce and Alfa Laval, the diffusion-bonded plate-fin exchanger is aimed at applications that require high pressure capabilities and the ability to handle seawater as the cooling medium. Currently available only in titanium, it operates at temperatures to 750 degrees Celsius, and pressures to 500 bars.

Designed for high-pressure gas services on offshore platforms, namely gas compression coolers in the compressor train, the plate-fin model can also be used for high-pressure crude exchangers and gas dehydration duties. The unit can also handle multiple-stream flows, with the number of streams limited only by the size of the equipment. The exchanger is a fraction of both the size and weight of conventional exchangers, which, on an offshore platform, have a large impact on costs.

The large channel size -- up to 5 mm -- means that relatively dirty services, such as raw wellhead gas and seawater, can be handled without plugging problems. Since the channels are integrally bonded together throughout the structure of the exchanger, it is inherently more reliable than tubular exchangers, which can suffer catastrophic failure should a tube rupture.

Printed-circuit model. Aptly named, because the channel-forming method mimics that used to manufacture printed-circuit boards for electronic components, the printed-circuit diffusion-bonded exchanger has been produced for several years in stainless steel, and other iron and nickel alloys. Channels of 0.5-2 mm are chemically etched into flat sheets, which are then diffusion-bonded together, with the connections welded.

Light and compact, the unit can handle pressures to 400 bars and temperatures to 900 degrees Celsius. Feed-effluent exchange, two-phase processes and other applications that lie outside the range of gasketed plate exchangers can be accommodated with this model. Like several other compact exchangers, it can be designed for multiple-pass and multiple-stream construction. However, the unit should only be used for clean services, such as clean-gas cooling, because of its very small channel size. Cooling water must be adequately treated, so that neither scaling nor biological growth plugs the channels.

Diffusion-bonded Heat exchangers ,Printed-circuit model operation Temperature ranges

Diffusion-bonded Heat exchangers 

Technological innovations continue to extend the range of compact heat exchangers, pushing the temperature threshold to 1,000 degrees Celsius and pressure to 500 bars, depending on the material of construction and type of exchanger. Diffusion bonding creates a block of ``parent metal'' with uniform grain structure throughout the heat exchanger, even across what were once individual plates. The result is a unit with no joints or seams other than the external connection attachments.

Plate-fin model. Produced by a joint venture of Rolls Royce and Alfa Laval, the diffusion-bonded plate-fin exchanger is aimed at applications that require high pressure capabilities and the ability to handle seawater as the cooling medium. Currently available only in titanium, it operates at temperatures to 750 degrees Celsius, and pressures to 500 bars.

Designed for high-pressure gas services on offshore platforms, namely gas compression coolers in the compressor train, the plate-fin model can also be used for high-pressure crude exchangers and gas dehydration duties. The unit can also handle multiple-stream flows, with the number of streams limited only by the size of the equipment. The exchanger is a fraction of both the size and weight of conventional exchangers, which, on an offshore platform, have a large impact on costs.

The large channel size -- up to 5 mm -- means that relatively dirty services, such as raw wellhead gas and seawater, can be handled without plugging problems. Since the channels are integrally bonded together throughout the structure of the exchanger, it is inherently more reliable than tubular exchangers, which can suffer catastrophic failure should a tube rupture.

Printed-circuit model. Aptly named, because the channel-forming method mimics that used to manufacture printed-circuit boards for electronic components, the printed-circuit diffusion-bonded exchanger has been produced for several years in stainless steel, and other iron and nickel alloys. Channels of 0.5-2 mm are chemically etched into flat sheets, which are then diffusion-bonded together, with the connections welded.

Light and compact, the unit can handle pressures to 400 bars and temperatures to 900 degrees Celsius. Feed-effluent exchange, two-phase processes and other applications that lie outside the range of gasketed plate exchangers can be accommodated with this model. Like several other compact exchangers, it can be designed for multiple-pass and multiple-stream construction. However, the unit should only be used for clean services, such as clean-gas cooling, because of its very small channel size. Cooling water must be adequately treated, so that neither scaling nor biological growth plugs the channels.

Diffusion-bonded Heat exchangers,Plate-fin model operation temperature and pressure ranges

Diffusion-bonded Heat exchangers 

Technological innovations continue to extend the range of compact heat exchangers, pushing the temperature threshold to 1,000 degrees Celsius and pressure to 500 bars, depending on the material of construction and type of exchanger. Diffusion bonding creates a block of ``parent metal'' with uniform grain structure throughout the heat exchanger, even across what were once individual plates. The result is a unit with no joints or seams other than the external connection attachments.

Plate-fin model. Produced by a joint venture of Rolls Royce and Alfa Laval, the diffusion-bonded plate-fin exchanger is aimed at applications that require high pressure capabilities and the ability to handle seawater as the cooling medium. Currently available only in titanium, it operates at temperatures to 750 degrees Celsius, and pressures to 500 bars.

Designed for high-pressure gas services on offshore platforms, namely gas compression coolers in the compressor train, the plate-fin model can also be used for high-pressure crude exchangers and gas dehydration duties. The unit can also handle multiple-stream flows, with the number of streams limited only by the size of the equipment. The exchanger is a fraction of both the size and weight of conventional exchangers, which, on an offshore platform, have a large impact on costs.

The large channel size -- up to 5 mm -- means that relatively dirty services, such as raw wellhead gas and seawater, can be handled without plugging problems. Since the channels are integrally bonded together throughout the structure of the exchanger, it is inherently more reliable than tubular exchangers, which can suffer catastrophic failure should a tube rupture.

Printed-circuit model. Aptly named, because the channel-forming method mimics that used to manufacture printed-circuit boards for electronic components, the printed-circuit diffusion-bonded exchanger has been produced for several years in stainless steel, and other iron and nickel alloys. Channels of 0.5-2 mm are chemically etched into flat sheets, which are then diffusion-bonded together, with the connections welded.

Light and compact, the unit can handle pressures to 400 bars and temperatures to 900 degrees Celsius. Feed-effluent exchange, two-phase processes and other applications that lie outside the range of gasketed plate exchangers can be accommodated with this model. Like several other compact exchangers, it can be designed for multiple-pass and multiple-stream construction. However, the unit should only be used for clean services, such as clean-gas cooling, because of its very small channel size. Cooling water must be adequately treated, so that neither scaling nor biological growth plugs the channels.

Diffusion-bonded Heat exchangers operation Temperature and pressure rangers

Diffusion-bonded Heat exchangers 

Technological innovations continue to extend the range of compact heat exchangers, pushing the temperature threshold to 1,000 degrees Celsius and pressure to 500 bars, depending on the material of construction and type of exchanger. Diffusion bonding creates a block of ``parent metal'' with uniform grain structure throughout the heat exchanger, even across what were once individual plates. The result is a unit with no joints or seams other than the external connection attachments.

Plate-fin model. Produced by a joint venture of Rolls Royce and Alfa Laval, the diffusion-bonded plate-fin exchanger is aimed at applications that require high pressure capabilities and the ability to handle seawater as the cooling medium. Currently available only in titanium, it operates at temperatures to 750 degrees Celsius, and pressures to 500 bars.

Designed for high-pressure gas services on offshore platforms, namely gas compression coolers in the compressor train, the plate-fin model can also be used for high-pressure crude exchangers and gas dehydration duties. The unit can also handle multiple-stream flows, with the number of streams limited only by the size of the equipment. The exchanger is a fraction of both the size and weight of conventional exchangers, which, on an offshore platform, have a large impact on costs.

The large channel size -- up to 5 mm -- means that relatively dirty services, such as raw wellhead gas and seawater, can be handled without plugging problems. Since the channels are integrally bonded together throughout the structure of the exchanger, it is inherently more reliable than tubular exchangers, which can suffer catastrophic failure should a tube rupture.

Printed-circuit model. Aptly named, because the channel-forming method mimics that used to manufacture printed-circuit boards for electronic components, the printed-circuit diffusion-bonded exchanger has been produced for several years in stainless steel, and other iron and nickel alloys. Channels of 0.5-2 mm are chemically etched into flat sheets, which are then diffusion-bonded together, with the connections welded.

Light and compact, the unit can handle pressures to 400 bars and temperatures to 900 degrees Celsius. Feed-effluent exchange, two-phase processes and other applications that lie outside the range of gasketed plate exchangers can be accommodated with this model. Like several other compact exchangers, it can be designed for multiple-pass and multiple-stream construction. However, the unit should only be used for clean services, such as clean-gas cooling, because of its very small channel size. Cooling water must be adequately treated, so that neither scaling nor biological growth plugs the channels.