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.

If you are looking for more details, kindly visit our website.

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.

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.

Why Consider higher speed pumps?

The standard motor induction speeds that are most often considered for pumps are 900, , and 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 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 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. 

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.

Recommended article:
Questions And Answers About Flat Pack Containers(1) - CIMC TLC
The Ultimate Guide to Choosing grp water tank

If you want to learn more, please visit our website INDUX.

* 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.

Do not let a pump run at zero flow

Do not let a pump operate for long periods of time at zero flow. In residential systems, the pressure switch shuts the pump down when the pressure is high which means there is low or no flow.

Do not let a pump run dry, use a check valve

Most pumps cannot run dry, ensure that the pump is always full of liquid. In residential systems, to ensure that the pump stays full of the liquid use a check valve (also called a foot valve) at the water source end of the suction line. Certain types of centrifugal pumps do not require a check valve as they can generate suction at the pump inlet to lift the fluid into the pump.

Suction valves

Gate valves at the pump suction and discharge should be used as these offer no resistance to flow and can provide a tight shut-off. Butterfly valves are often used but they do provide some resistance and their presence in the flow stream can potentially be a source of hang-ups which would be critical at the suction. They do close faster than gate valves but are not as leak proof.

Eccentric reducer

Always use an eccentric reducer at the pump suction when a pipe size transition is required. Put the flat on top when the fluid is coming from below or straight (see next Figure) and the flat on the bottom when the fluid is coming from the top. This will avoid an air pocket at the pump suction and allow air to be evacuated.

Flow control

If you need to control the flow, use a valve on the discharge side of the pump, never use a valve on the suction side for this purpose.

Plan ahead for flow meters

For new systems that do not have a flow meter, install flanges that are designed for an orifice plate in a straight part of the pipe and do not install the orifice plate. In the future, whoever trouble-shoots the pump will have a way to measure flow without the owner having to incur major downtime or expense. Note: orifice plates are not suitable for slurries.

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. 

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

It seems like a hack exists for everything these days. From a quicker way to wash your car to unusual ways to remove wood scuffs, there’s a shortcut for doing just about all your everyday chores better, faster, or cheaper. What about at work, though? If you’re pumping slurry, KSB GIW, Inc. has got you covered.

Here are five hacks every pump operator should know:

1. Avoid cavitation the easy way. Hear a popping noise while your pump is operating? If it sounds like popcorn, you’re hearing vapor cavities imploding — and they’re as destructive as they are noisy. These tiny pockets of air are formed when the absolute pressure at the eye of the impeller reaches the vapor pressure of the liquid. Their continuous popping can result in costly damage to your impeller and bearings.

Avoiding cavitation is easy if you have the right pump. You need to know the required critical suction pressure — or Net Positive Suction Head (NPSH) — for your system and choose a slurry pump that operates clear of that pressure.
2. Determine your GIW cartridge bearing assembly’s oil level with a glance. If you’re curious about your oil level, all you have to do is find your oil level sight glass. Your oil level should be at the center line. Just remember: Never check the oil while the pump is running!
3. Install a snap ring gasket — in a snap! The snap ring gasket on a KSB GIW, Inc. LSA® pump seals the joint between the suction liner and pump casing. To replace it, make sure the part number is facing out, lubricate it using a non-petroleum-based lubricant such as dish soap or baby powder, and make sure it’s facing the right direction. Then, ensuring this trapezoid-shaped gasket is in the right spot is simple: Pull it up, and if it “snaps” back into place, it’s installed correctly.
4. Simplify impeller removal. Struggling to get your impeller off its shaft? When you have one gasket, the torque from start ups will cause it to stick to both the impeller and shaft, making it difficult to separate during maintenance. However, we’ve got a quick and easy fix: Simply use two aramid gaskets between the impeller and shaft sleeve instead of one. This way, one gasket will bond to the shaft and the other to the impeller, making removals quick and easy.

5. Make sure your nose gap is right on the nose. It’s may be a small maintenance task, but properly and regularly adjusting your nose gap can easily double or triple the wear life of your parts! The correct clearance, which is very small, allows the pump to maintain a high pressure at the outside of the impeller and a low pressure at the eye. However, as the pump operates, this nose gap increases incrementally, disrupting the pressure distribution and encouraging particles to recirculate around the impeller suction shroud.

To save the wear and tear on your impeller and suction liner, adjust the nose gap once a week. It’s ultra quick: Start by loosening the hold-down bolts on the back of the bearing assembly, then use your adjusting screw and nut to move the bearing assembly forward until it touches the liner, taking care to back it off slowly to the prescribed distance for your pump. Voilà — the perfect nose gap!

Pump hacks like these may require outside-the-box thinking, but that’s what makes them great! At KSB GIW, Inc., we’re always trying to come up with new ways to simplify your pumping processes and maintenance procedures. With a little creative guidance — and a great pump — you’ll be operating longer and more cost-effectively, so you’ll have more time to focus on production — and those wood scuffs!