Slurry Pump FAQ’S

How to Reduce the pump’s NPSHr?

1. Use a pump with a larger suction flange. This lowers the Hi. An example of this would be to
change a 3 • 4 • 10 pump into a 4 • 6 • 10 pump. The 10-inch impeller needs to remain the
same for discharge pressure. However, by converting the 4-inch suction flange into a 6-inch
suction flange, the inlet losses would be reduced.

2. Machine and polish the suction throat of the pump. This is probably the worst casting, and
roughest finish in the entire pump. Center the suction flange on a lathe and ream-out the
suction throat. This reduces the Hi.

3. Machine open and increase the inside diameter of the eye of enclosed impellers. This reduces
the Hi.

4. Use a larger/slower pump. This reduces the Hi and Hf.

5. Use a small booster pump to feed the principle pump. This increases the artificial head (Ha).

6. Use smaller capacity pumps in parallel. This reduces the Hi and Hf.

7. Use a double suction impeller. Convert an end suction centrifugal pump into a split case horizontal design.

8. Use an impeller inducer.

How to Increase the Pumps NPSHa?

1. Raise the level in the suction vessel. This increases Hs.

2. Elevate the suction vessel. This increases Hs.

3. Lower the pump. This increases Hs.

4. Reduce the friction in the suction piping. This is probably the most creative way to deal with cavitation. This reduces the Hr.
a. Use larger diameter suction pipe.
b. Change the pipe schedule. If there is a designated schedule, you can bet it was based on
discharge pressures and not suction pressures.
c. Change to pipe with lower friction characteristics. Ex. Change cast iron piping for PVC or
even food grade stainless.
d. Move the pump closer to the suction vessel.
e. Convert globe valves into gate valves if possible.
f. Convert quarter turn butterfly valves into ball valves.
g. Be sure all ball valves are full port design.
h. Be sure all suction valves are totally open.
i. Reduce multiple elbows. If a system is designed with 9 or more elbows in the suction
piping, you can be sure that some of these elbows are self-canceling. If so, then some
elbows can be eliminated.
j. Convert 2 or 3 close fitting elbows into a flexible ‘S’.
k. Convert ‘mitered 90 ~ elbows’ and short radius elbows into long radius elbows.
I. Change filters and strainers with more frequency.
m. Be sure all pipe gaskets and ring seals are perfectly centered within the flange faces.
n. Keep suction pipe inside diameters clean and scale free.

5. Lower the temperature of the fluid in the suction vessel. This decreases the Hvp.

6. Pressurize the suction vessel. This increases the artificial Ha 23 ft for every 10 psi.

How to Determine Net Positive Suction Head Available (NPSHa)?

NPSHa is the energy of fluid at the suction and should be greater than NPSHr. Generally, the NPSHa should be at least 10% or 3ft above NPSHr (whatever is greater). But to be safe, it is good practice to be 50% NPSHr, to avoid incipient cavitation.

NPSHa =Ha + Hs – H1·p – Hf – Hi:

Where:
Ha = atmospheric head (14.7 psi x 2.31) = 33.9 ft at sea level or lookup on table
Hs = static head in feet (positive or negative) of the fluid level
Hvp = vapor head of the fluid (lookip on table)
Hf = Friction head or friction losses in suction piping/connections
Hi = inlet head, losses in the suction throat (i.e. eye of impellor) these may be insignificant or substantial

What is a pump characteristic curve?

The performance or characteristic curve of the pump provides information on the relationship between total head and flow rate. There are three important points on this curve.

1. The shut-off head, this is the maximum head that the pump can achieve and occurs at zero flow. The pump will be noisy and vibrate excessively at this point. The pump will consume the least amount of power at this point. See also the pump glossary.

2. The best efficiency point B.E.P. this is the point at which the pump is the most efficient and operates with the least vibration and noise. This is often the point for which pump’s are rated and which is indicated on the nameplate. The pump will consume the power corresponding to its B.E.P. rating at this point.

3. The maximum flow point, the pump may not operate past this point. The pump will be noisy and vibrate excessively at this point. The pump will consume the maximum amount of power at this point.

Sometimes the characteristic curve will include a power consumption curve. This curve is only valid for water, if the fluid has a different density than water you cannot use this curve. However you can use the total head vs. flow rate curve since this is independent of density.

If your fluid has a different viscosity than water you cannot use the characteristic curve without correction. Any fluid with a viscosity higher than 10 cSt will require a correction. Water at 60F has a viscosity of 1 cSt.

Pump Normal, flat and drooping characteristic curves

There are three different characteristic curve profiles for radial flow pumps. Figure 4 shows the various vane profiles that exist and the relationship between them. This tip is related to the radial vane profile which is the profile of the typical centrifugal pump.

What is the meaning of specific speed for a Pump?

If you are having trouble with a pump or want to check whether the new pump to be installed is appropriate, check the specific speed and the suction specific speed of the pump. The specific speed provides a number which can help identify the type of pump (for example radial or axial flow) that is best suited for your application. The specific speed of the pump type selected (see Figure 4) should be close to the specific speed calculated for your application. The suction specific speed will tell you if the suction of the pump is likely to cause problems in your application.

Why Consider higher speed pumps?

The standard motor induction speeds that are most often considered for pumps are 900, 1200, 1800 and 3600 rpm. The specific speed of a pump depends on its speed and the higher the specific speed the higher the efficiency.

By choosing a higher speed pump the pump will be smaller and maybe less expensive. Modern anti-friction bearings are quite capable of handling these higher speeds without compromising their useful life. Will there be increased wear within the casing? Probably not since for the same head the fluid particle speed within the casing will be the same because of the smaller impeller diameter turns at a higher rpm giving roughly similar linear velocities.

Therefore, if you want to conserve energy and reduce costs always check to see if a pump running at a higher speed can meet your requirement. However, you should check the suction specific speed number and make sure it is below 11000 to avoid problems with cavitation.

In a multiple and identical pump system, if one pump is in poor running order what is the effect on the discharge header head and the flow to the system?

Consider a two pump system where one of the pumps is in poor running condition as compared to the other. This could be due to: worn or damaged impeller, worn casing, worn bearings and shaft, wrong impeller, etc. Any or a combinations of these factors will have an effect on the pump’s performance. The efficiency of the pump will be affected as well as the head and flow. It is difficult to predict the resulting performance curve without doing tests. However, unless the pump has gaping holes, the performance curve should look similar to that of the good pump but with a lower capacity and head. Let’s assume that there is negligible friction loss between the discharges of pumps A or B and the header and also the head at the inlet of both pumps is the same. The operating point is point 1 on curve A which corresponds to 500 USGPM and 96 ft. The curve for the bad pump B, being slightly lower will contribute 265 USGPM at 96 ft since it must operate at the same head as pump A. Therefore, the total flow will be 765 USGPM. If we had two good pumps the total flow would be 1000 USGPM instead of 765 USGPM. The head is not affected since it is the pump with the higher head which will control the pressure head in the discharge header by forcing the other pump to reduce its flow to match the higher pressure head. This what is meant when people say that one pump is fighting the other. Improperly designed suction or discharge piping can have this effect also. 

What is Pump Head Available?

NPSHa ≥ NPSHr

You must configure your system so NPSHa ≥ NPSHr (the head available from the system is greater than the head the pump requires).
Failure to meet this requirement will cause reduced flow rate, cavitation, and vibration of the pump.
NPSHa = Ha + Hs – Hv – Hf
Ha = pressure on the liquid surface in the supply tank
Hs = suction head (+) or suction lift (-)
Hv = vapour pressure
Hf = friction loss in the suction piping

Minimum flow rate

Most pumps should not be used at a flow rate less than 50% of the B.E.P. (best efficiency point) flow rate without a recirculation line. (What is the B.E.P.?) If your system requires a flow rate of 50% or less then use a recirculation line to increase the flow through the pump keeping the flow low in the system, or install a variable speed drive.

The factors which determine minimum allowable rate of flow include the following:

* Temperature rise of the liquid — This is usually established as 15°F and results in a very low limit. However, if a pump operates at shut off, it could overheat badly.

* Radial hydraulic thrust on impellers — This is most serious with single volute pumps and, even at flow rates as high as 50% of BEP could cause reduced bearing life, excessive shaft deflection, seal failures, impeller rubbing and shaft breakage.

* Flow re-circulation in the pump impeller — This can also occur below 50% of BEP causing noise, vibration, cavitation and mechanical damage.

* Total head characteristic curve – Some pump curves droop toward shut off, and some VTP curves show a dip in the curve. Operation in such regions should be avoided.

How Does Air in a Pump Reduce Capacity?

When air enters a pump it sometimes gets trapped in the volute, this reduces the capacity, creates vibration and noise. To remedy, shut the pump down and open the vent valve to remove the air. If the pump is excessively noisy do not automatically assume that the problem is cavitation, air in the pump creates vibration and noise. Cavitation produces a distinct noise similar to gravel in a cement mixer.

What Effect does viscosity on pump performance?

Viscosity is the main criteria which determines whether the application requires a centrifugal pump or a positive displacement pump. Centrifugal pumps can pump viscous fluids however the performance is adversely affected. If your fluid is over 400 cSt (centiStokes) in viscosity consider using a positive displacement pump.

Avoid pockets and high points

Avoid pockets or high point where air can accumulate in the discharge piping. An ideal pipe run is one where the piping gradually slopes up from the pump to the outlet. This will ensure that any air in the discharge side of the pump can be evacuated to the outlet.

Location of control valves

Position control valves closer to the pump discharge outlet than the system outlet. This will ensure positive pressure at the valve inlet and therefore reduce the risk of cavitation.

When the valve must be located at the outlet such as the feed to a tank, bring the end of the pipe to the bottom of the tank and put the valve close to that point to provide some pressure on the discharge side of the valve making it easier to size the valve, extending it’s life and reducing the possibility of cavitation.

The right pipe size

The right pipe size is a compromise between cost (bigger pipes are more expensive) and excessive friction loss (small pipes cause high friction loss and will affect the pump performance).

Generally speaking, the discharge pipe size can be the same size as the pump discharge connection, you can see if this is reasonable by calculating the friction loss of the whole system. For the suction side, you can also use the same size pipe as the pump suction connection, often one size bigger is used A typical velocity range used for sizing pipes on the discharge side of the pump is 9-12 ft/s and for the suction side 3-6 ft/s.

Pressure at high point of system

Calculate the level of pressure of the high point in your system. The pressure may be low enough for the fluid to vaporize and create a vapor pocket which will be detrimental to the performance of the system. The pressure at this point can be increased by installing a valve at some point past the high point and by closing this valve you can adjust the pressure at the high point. Of course, you will need to take that into account in the total head calculations of the pump.

Pump pressure rating and series operation

For series pump installations make sure that the pressure rating of the pumps is adequate. This is particularly critical in the case where the system could become plugged due to an obstruction. All the pumps will reach their shut-of head and the pressure produced will be cumulative. The same applies for the pressure rating of the pipes and flanges.

Avoid running pump in reverse direction

Avoid running a pump in reverse direction, pump shafts have been broken this way especially if the pump is started while running backwards. The simplest solution is to install a check valve on the discharge line. 

Pump Cavitation

Formation and collapse of low-pressure cavities in a flowing liquid. As liquid pressure is reduced below vapor pressure by either static or dynamic means, vapor-filled cavities form. In seawater, cavitation can occur at pressures slightly above vapor pressure. Cavitation bubbles form in areas of low pressure and collapse (implode). The process takes only a few milliseconds.

Effects of Solids on Pump Performance:

1. Head in meters of mixture parallels clear water except in reduction due to the presence of solids.
2. Pressure obtained for water is increased in proportion to the specific gravity of the mixture
3. BHP increases in proportion to SG of the mixture (when viscosity does not increase)
4. Pipe friction loss expresses in meters of the mixture is essentially the same as for water
5. Pump Best Efficiency Point (BEP) remains the same as for water
6. Efficiency for a pump with slurry is reduced by the same radio as the head reduction below that for water.

Pump Performance Key Topics

Angular momentum and conservation of mass, flow rates, head, friction losses, power gained by the fluid/shaft power driving pump, typical centrifugal pump curves, pump efficiency, net positive suction head (NPSH), system curves, pump operating point max-efficiency, pumps in series and parallel, adding pumps in series add heads, pumps in parallel adds flowrate, power and efficiency math, water horsepower, brake horsepower, optimum efficiency related to vane design, dimensionless parameters & curves, pump affinity/scaling laws, the effect of density & viscosity on the pump, the effect of solids on pump performance, mixture curve, solids curves, head reduction factor, considerations for pumps in series (phased startup to minimize the maximum pressure), shell types, impeller types, gas accumulator for gassy sediments, flow meters, dredge automation (valves and indicators), Hofer valve to avoid cavitation, pump cavitation.

MAXIMUM PERCENT SOLIDS VS. CAVITATION

The maximum percent solids are defined as the highest practical, instantaneous percent of solids the hydraulic system can transport, without cavitating the pump.

Maximum percent solids occur at a point easily recognizable by the dredge leverman. On a conventional hydraulic dredge, i.e., with a pump in the hull but no submerged pump, the leverman sets his pump speed to achieve the desired velocity in the pipeline. This results in perhaps a 7- to 10-inch mercury reading on the vacuum gauge, representing the head or pressure losses in the suction line on water alone. Then as the operator lowers his cutter (excavator) into the bottom material, the vacuum rises abruptly because of the demand for the heavy solids. At some point, perhaps 24 to 27 inches mercury, the pump becomes noisy, vibrates, and loses its pumping effectiveness because of a phenomenon called cavitation. This occurs when the natural barometric pressure is no longer capable of overcoming the losses in the suction line at the rate the pump demands. The careful operator will control the pickup of solids so as to keep his vacuum indication an inch or two of mercury below the cavitation point. This, then, is the maximum percent solids that coincide with the maximum instantaneous production rate.

Sediment Transfer in Pipes

Pipeline transport is one the most efficient methods for moving large amounts of solids over long distances. Typical pipeline diameters in dredging range from 6 to 36 inches. Pipelines are commonly constructed of steel or a composite material such as polyethylene, (HDPE). They can be submerged, floating, or placed onshore. Pipeline length varies from several thousand feet to several miles. In general, larger solids will require more energy (higher pipeline pressure and more pump head) for transport. The effect of particle size can be dramatic. Very fine solids (clays) can alter the fluid viscosity if present in high enough concentration. Some soil types may form temporary structures that behave like larger particles in the pipeline (e.g., clay balls). Different types of solids and soils that might be transferred are cobbles, gravels including coarse, medium, and fines, sands, silt, clays, peats, and organics. In settling slurries the turbulence in the flow moves the solids. It is the eddies in the turbulence that moves the solids and is dependent upon velocity, if there is not enough velocity then solids will deposit and clog the slurry pipeline. The addition of finer solids help support and keep heavy solids, such as stones, from settling and clogging the slurry pipeline.

The flow velocity at which a deposit forms will vary with these variables

1. Particle diameter
2. Particle density
3. Pipeline diameter
4. Carrier fluid viscosity