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A Beginner's Guide To Pumping Slurry

13 May.,2024

A Beginner's Guide To Pumping Slurry

Centrifugal Pumps for Slurry

Slurry is one of the most challenging fluids to move. It's highly abrasive, thick, sometimes corrosive, and contains a high concentration of solids. No doubt about it, slurry is tough on pumps. But selecting the right centrifugal pump for these abrasive applications can make all the difference in the long-term performance.

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WHAT IS A "SLURRY"?

Slurry is any mixture of fluid and fine solid particles. Examples of slurries would include: manure, cement, starch, or coal suspended in water. Slurries are used as a convenient way to handle solids in mining, steel processing, foundries, power generation, and most recently, the Frac Sand mining industry.

Slurries generally behave the same way as thick, viscous fluids, flowing under gravity, but also pumped as needed. Slurries are divided into two general categories: non-settling or settling.

Non-settling slurries consist of very fine particles, which give the illusion of increased apparent viscosity. These slurries usually have low wearing properties, but do require very careful consideration when selecting the right pump because they do not behave in the same manner as a normal liquid does.

Settling slurries are formed by coarse particles that tend to form an unstable mixture. Particular attention should be given to flow and power calculations when selecting a pump. The majority of slurry applications are made up of coarse particles and because of this, have higher wear properties.

Below are common characteristics of slurries:

Abrasive
Thick consistency
Can contain a high amount of solids
Usually settle quickly
Require more power to operate than a “water” pump

SLURRY PUMP SELECTION

Many types of pumps are used for pumping slurries, but the most common slurry pump is the centrifugal pump. The centrifugal slurry pump uses the centrifugal force generated by a rotating impeller to impact kinetic energy to the slurry, similar to how a water-like liquid would move through a standard centrifugal pump.

Slurry applications greatly reduce the expected wear life of pumping components. It’s critical that pumps designed for these heavy-duty applications are selected from the start. Consider the following when making selections:

BASIC PUMP COMPONENTS

To ensure the pump will hold up against abrasive wear, the impeller size/design, material of construction, and discharge configurations must be properly selected.

Open impellers are the most common on slurry pumps because they’re the least likely to clog. Closed impellers on the other hand are the most likely to clog and the most difficult to clean if they clog.

Slurry impellers are large and thick. This helps them operate longer in harsh slurry mixtures.

SLURRY PUMP CONSTRUCTION

Slurry pumps are generally larger in size when compared to low-viscosity liquid pumps and usually require more horsepower to operate because they're less efficient. Bearings and shafts must be more rugged and rigid as well.

To protect the pump’s casing from abrasion, slurry pumps are oftentimes lined with metal or rubber. Goulds Pumps, for example, lines their XHD (Extra Heavy Duty) slurry pump with rubber.

Metal casings are composed of hard alloys. These casings are built to withstand the erosion caused by increased pressure and circulation.

The casings are selected to suit the needs of the application. For instance, pumps used in cement production handle fine particles at low pressures. Therefore, a light construction casing is acceptable. If the pump is handling rocks, the pump casing and impeller will need a thicker and stronger casing.

SLURRY PUMPING CONSIDERATIONS

Those with experience pumping slurries know it's not an easy task. Slurries are heavy and difficult to pump. They cause excessive wear on pumps, their components, and are known to clog suction and discharge lines if not moving fast enough.

It’s a challenge to make slurry centrifugal pumps last for a reasonable amount of time. But, there are a few things you can do to extend the life of your slurry pump and make pumping slurry less of a challenge.

Find the sweet spot that allows the pump to run as slow as possible (to reduce wear), but fast enough to keep solids from settling and clogging the lines
To reduce wear, lower the pump’s discharge pressure to the lowest point possible
Follow proper piping principles to ensure a constant and uniform delivery of the slurry to the pump

Pumping slurries poses several challenges and problems, but with proper engineering and equipment selection, you can experience many years of worry-free operation. It's important to work with a qualified engineer when selecting a slurry pump because slurries can wreak havoc on a pump if not properly selected.

Check out the Must-Have Handbook for Centrifugal Pumps for more information on centrifugal pumps, including details about pumps specifically designed for slurry applications!

Expert Centrifugal Pump Insights from Crane Engineering

Engineers and experts rely on Crane Engineering for insight and help with centrifugal pumps to pump slurry. Our in-house team of engineers can answer questions related to not only pumps but valves and skid systems. We provide a complete service and repair team who will fix pumps back to OEM standards. We are ready to assist you, contact us, today if you're in Wisconsin, Minnesota, Iowa, North Dakota, South Dakota, and the Upper Peninsula of Michigan. Crane Engineering also builds and designs customized skid systems nationwide.

Let us know how we can help you, request a quote today!

Troubleshooting For Slurry Pumps

by Bill Noakes

A guide to common slurry pump and gland problems and how you can overcome them.

Symptoms - Hopper Overflows

Possible causes:

- Pump not primed
- Excessive amount of air or gas in liquid
- Inlet of suction pipe insufficiently submerged
- Blocked suction line
- Suction valve closed
- Inlet pipe diameter too small or length of inlet pipe too long
- Pump speed too slow or wrong direction of rotation
- Coupling or drive belts not transmitting power
- Total head of system higher than design head
- Specific gravity of liquid different from design
- Entrained air in pump, pump hopper requires baffles
- Badly installed pipeline or gaskets partly blocking pipe
- Foreign matter in impeller

Symptoms - Overheating or Seizure of Pump

Possible causes:

- Pump not primed
- Insufficient margin between suction pressure and vapour pressure
- Operation at very low capacity
- Drive misalignment
- Lack of lubrication
- Improper installation of bearings
- Rotating part rubbing on stationary part
- Impeller out of balance, resulting in vibration
- Excessive thrust caused by a mechanical failure inside the pump
- Excessive amount of lubricant in bearing housing causing high bearing temp

Symptoms - Short Bearing Life

Possible causes:

- Drive misalignment
- Shaft bent
- Rotating part rubbing on stationary part
- Bearings worn
- Impeller out of balance, resulting in vibration
- Shaft running off-centre because of worn bearings or misalignment
- Lack of lubrication
- Excessive amount of lubricant in bearing housing causing high bearing temp
- Improper installation of bearings
- Dirt getting into bearings
- Rusting of bearings due to water getting into housing

Symptoms - Vibration and noise from Pump

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Possible causes:

- Pump or suction pipe not completely filled with liquid
- Suction lift too high
- Insufficient margin between suction pressure and vapour pressure
- Foot valve too small
- Foot valve partially clogged
- Inlet of suction pipe insufficiently submerged
- Operation at very low capacity
- Badly installed pipe line or gaskets partly blocking pipe
- Drive misalignment
- Rotating part rubbing on stationary part
- Shaft running off-centre because of worn bearings or misalignment
- Foreign matter in impeller

Symptoms - Packing has short life

Possible causes:

- Misalignment
- Shaft bent
- Shaft or shaft sleeves worn or scored at the packing
- Packing improperly installed
- Shaft running off-centre because of worn bearings or misalignment
- Gland sealing water flow inadequate due to worn shaft seal components
- Gland sealing water pressure inadequate
- Dirt or grit in sealing liquid, leading to scoring shaft sleeve
- Expeller worn or blocked
- Excessive clearance at bottom of stuffing box, forcing packing into pump

Symptoms - Leakage from the stuffing box

Possible causes:

- Misalignment
- Shaft bent
- Shaft or shaft sleeves worn or scored at the packing
- Packing wrong size
- Packing improperly installed
- Incorrect type of packing for operating conditions
- Shaft running off-centre because of worn bearings or misalignment
- Gland sealing water pressure too high causing packing extrusion
- Dirt or grit in sealing liquid, leading to scoring shaft sleeve
- Expeller worn or blocked
- Excessive clearance at bottom of stuffing box, forcing packing into pump

Symptoms - Excessive motor power required

Possible causes:

- Speed too high
- Wrong direction of rotation
- Total head of system higher than design
- Total head of system lower than design
- Specific gravity of liquid different from design
- Viscosity of liquid differs from that for which designed
- Misalignment
- Shaft bent
- Rotating part rubbing on stationary part
- Packing improperly installed
- Gland too tight, resulting in no flow of liquid to lubricate packing

Symptoms - Pump loses prime

Possible causes:

- Pump or suction pipe not completely filled with liquid
- Suction lift too high
- Excessive amount of air or gas in liquid
- Air pocket in suction line
- Air leaks into suction line
- Air leaks into pump through stuffing box
- Inlet of suction pipe insufficiently submerged
- Blocked suction line
- Foreign matter in impeller
- Inlet pipe diameter too small or length of inlet pipe too long
- Entrained air in pump. Pump hopper requires baffles
- Badly installed pipe line or gaskets partly blocking pipe

Symptoms - Insufficient pressure

Possible causes:

- Excessive amount of air or gas in liquid
- Speed too slow
- Total head of system higher than design
- Viscosity of liquid differs from that for which designed
- Entrained air in pump. Pump hopper requires baffles
- Badly installed pipe line or gaskets partly blocking pipe
- Impeller damaged or worn
- Casing gasket defective, permitting internal leakage
- Foreign matter in impeller

Symptoms - Reduced discharge delivery

Possible causes:

- Pump or suction pipe not completely filled with liquid
- Suction lift too high
- Insufficient margin between suction pressure and vapour pressure
- Excessive amount of air or gas in liquid
- Air pocket in suction line
- Air leaks into suction line
- Air leaks into pump through stuffing box
- Foot valve too small
- Foot valve partially clogged
- Inlet of suction pipe insufficiently submerged
- Blocked suction line
- Inlet pipe diameter too small or length of inlet pipe too long
- Speed too slow - slippage of vee belts
- Speed too slow - incorrect size pulleys
- Speed too slow - incorrect "pole" of electric motor
- Wrong direction of rotation
- Total head of system higher than design
- Viscosity of liquid differs from that for which designed
- Entrained air in pump. Pump hopper requires baffles
- Badly installed pipe line or gaskets partly blocking pipe
- Foreign matter in impeller

Symptoms - Failure to discharge

Possible causes:

- Pump not primed
- Pump or suction pipe not completely filled with liquid
- Suction lift too high
- Insufficient margin between suction pressure and vapour pressure
- Air pocket in suction line
- Inlet of suction pipe insufficiently submerged
- Blocked suction line
- Inlet pipe diameter too small or length of inlet pipe too long
- Speed too slow
- Total head of system higher than design
- Impeller damaged or worn
- Foreign matter in impeller

Pump Troubleshooting - Glands

Steps for the correct fitting of packing into the stuffing box

1. Packing is normally available in pre-cut and formed rings, or alternatively the user can cut rings from a roll.

2. It is critical that each packing ring is cut to the correct length.

3. The packing cut should be at an angle of 45°, ie, a scarf joint.

4. The packing should fill the annulus. A length of key steel or similar should be used to push the packing into the stuffing box. Care should be taken to start fitting the packing on the joint side and then work around the ring to the opposite side. This should prevent stretching the ring, which could lead to overlap at the cut.

5. The packing should push in easily. If not, check the section dimensions. If incorrect the packing should be replaced. Do not roll formed rings.

6. Each packing should be tamped into position before fitting the next packing. Optimum arrangements for multiple rings of packing are shown below.

7. When all packing rings have now been fitted, assemble the gland and nip the bolts to flatten packing rings evenly. Do not tighten fully.

8. Run the pump. If excessive leakage occurs, tighten the bolts slightly and leave for at least an hour. Repeat if excessive leakage continues. Note that a good steady even drip or constant stream is desirable to cool and lubricate the gland

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