Centrifugal Pumps

Centrifugal: "Moving or directed away from the center (or axis)"

Centrifugal pumps are the most common type of pump used in plumbing systems. This article explores the basic design concepts and functional principals of these pumps.

Fig 1 illustrates a cross-section of a typical centrifugal pump.

Fluid enters the inlet port at the center of the rotating impeller, or the suction eye.

As the impeller spins in a counter-clockwise direction, it thrusts the fluid outward radially, causing centrifugal acceleration.

As it does this, it creates a vacuum in its wake, drawing even more fluid into the inlet.

Centrifugal acceleration creates energy proportional to the speed of the impeller. The faster the impeller rotates, the faster the fluid movement and the stronger its force. This energy is harnessed by introducing resistance.

Remember, a pump does not create pressure; it only provides flow.
Pressure is a measure of the amount of resistance to that flow.




A centrifugal pump has two main components, one moving and one stationary.


The moving component consists of an impeller and a shaft.


The stationary component consists of a casing, cover, and bearings. These are illustrated at the left, in Fig 2.




Moving Components: Impellers & Shafts

Impeller
Impellers are the rotating blades that actually move the fluid. They are connected to the drive shaft that rotates within the pump casing. The impeller is designed to impart a whirling or motion to the liquid in the pump.

Impellers are classified in a number of different ways:

1. Direction of flow relative to the axis of the shaft.
   - Radial flow
   - Axial flow
   - Mixed flow
   
   
2. Type of suction
   - Single-suction (Liquid inlet on one side)
   - Double-suction (Liquid inlet on both sides)
   
   
3. Mechanical construction (FIG 3)
   - Open: No shrouds or wall to enclose the vanes
   - Closed: Shrouds or sidewall enclosing the vanes
   - Semi-open or vortex type.
   
   Open and semi-open impellers are less prone to clogging, but require manual adjustment to the volute or back-plate to prevent internal re-circulation.
   
   Closed impellers require wear rings, which must be replaced periodically, presenting a maintenance problem.
   
   Vortex impellers are effective for solids and fibrous materials but they are less efficient than other designs.
   

Stages
The number of impellers determines the number of stages of the pump.

• SingleStage pump has just one impeller and is better for low head service
• Two-Stage pumphas two impellers mounted in series for medium head service.
• Multi-Stage pump has three or more impellers mounted in series for high head service such as in deep well pumps.

Impeller Class (Shape)
Specific Speed is used to classify pump impellers as to their type and proportions.

It is defined as the speed in RPMs (revolutions per minute) at which a similar impeller would have to operate in order to deliver one gallon per minute flow against one-foot head.

The specific speed determines the general shape or class of the impellers.

Radial flow impellers develop head principally through centrifugal force. Radial impellers are generally used in low flow high head designs, while Axial impellers are used in high flow low head designs. Pumps of higher specific speeds develop head partly by centrifugal force and partly by axial force.

Shafts
Shaft Sleeves extend beyond the outer face of the seal gland plate and protect the shafts from erosion, corrosion, and wear.

Leakage between the shaft and the sleeve is different
from leakage through the mechanical seal

Shaft Couplings compensate for axial growth of the shaft and transmit torque to the impeller. They may be either rigid or flexible.

Rigid couplings are used when there is chance of misalignment. Flexible couplings are more forgiving, and may be either elastomeric (using rubber or polymer parts) or non-elastomeric (using metallic parts).


Stationary Components: Casing

Casing
The pump casing creates the first resistance. The liquid decelerates still more in the discharge nozzle, where its velocity is converted to pressure

Casings are generally either volute or circular.

A volute is a curved funnel that increases in size to the discharge port. As its size increases, the volute reduces the speed of the liquid and increases the output pressure.

This helps to balance hydraulic pressure on the shaft; however, running volute-style pumps at slow speeds puts undue stress on the shaft, which in turn increases wear-and-tear on the seals and bearings, as well as on the shaft itself.

Volute casings build a higher head, but circular casings are generally used on higher capacity pumps. Circular casings have stationary diffusion vanes around the impeller that convert velocity energy to pressure energy. Diffusers are typically used in multi-stage pumps.

Casings are either solid or split.

In a solid casings design the entire casing is in one piece. Split casings are made of two or more parts fastened together, split either horizontally (axially split), or vertically (radially split). Wear rings seal the casing from the impeller.

Suction and discharge nozzles are built into the casings. They are typically made in one of the following ways:

• End suction/Top discharge. The discharge nozzle is perpendicular to the shaft.
• Top suction /Top discharge. Both nozzles are perpendicular to the shaft. This pump is always a radially split case pump.
• Side suction / Side discharge. Both nozzles are perpendicular to the shaft. This pump can have either an axially or radially split case type.
  

The space between the shaft and casing is called the chamber.

If a mechanical seal is used in the pump, the chamber is commonly referred to as a Seal Chamber.

If packing is used to form the seal, the chamber is referred to as a Stuffing Box.

Both the seal chamber and the stuffing box protect the pump against leakage where the shaft passes through the casing. They also maintain proper temperature control.

An adjustable gland helps the packing or the seal fit properly on the shaft sleeve. The throat or throttle bushing forms a close clearance around the sleeve. An internal circulating device (pumping ring) circulates fluid through a cooler or reservoir.

Bearings
The bearing housing encloses the bearings that keep the shaft in correct alignment with the stationary parts. It also includes an oil reservoir for lubrication, oiler, and cooling jacket.

Auxiliary Components
Auxiliary components generally include seal drains, vents, and cooling systems, bearing lubrication, seal chamber or stuffing box cooling, and pump pedestal cooling systems.

Auxiliary piping systems may include tubing, piping, various types of valves and gauges, thermocouples, sight flow indicators, fluid reservoirs, and all related vents and drains.

Pump Capacity
Capacity is the flow rate in gallons per minute (GPM) at which liquid is moved or pushed by the pump.

Capacity depends on the pressure, temperature, and viscosity of the liquid being pumped, the size of the pump and the shape of the cavities between the vanes, and on the size and speed of the impeller.

Note: Pressure output of pumps is measured as "feet of head" rather than "pounds per square inch"


Since liquids are essentially incompressible, capacity is directly related with the velocity of flow in the suction pipe. This relationship is as follows:
       Q = 449 x V x A

        Q = Capacity in gallons per minute
        V = Velocity in flow in feet per second
        A = Area of pipe in square ft.

Power and Efficiency
Brake Horsepower (BHP) is the actual horsepower delivered to the pump shaft, defined as follows:

       BHP = Q x Hr x Sp. Gr.
              3960 x Eff.

        Q = Capacity in gallons per minute
        Hr = Total Differential Head in absolute feet
        Sp. Gr. = Specific Gravity of the liquid.
        Eff. = Pump efficiency as a percentage

Water Horsepower (WHP) is the hydraulic horsepower delivered by the pump, defined as follows:

      WHP = Q x Hr x Sp/Gr.
                  3960

        Q = Capacity in gallons per minute
        Hr = Total Differential Head in absolute feet
        Sp. Gr. = Specific Gravity of the liquid

The constant (3960) is the number of foot-pounds in one horsepower (33,000) divided by the weight of one gallon of water (8.33 pounds).

Brake horsepower is always greater than hydraulic horsepower due to the friction in the pump. Pump efficiency is the ratio of these two values.
Pump Efficiency = WHP
BHP

Best Efficiency Point (BEP) is the capacity at maximum impeller size at which the efficiency is highest.

All points above or below BEP have a lower efficiency, and the impeller is subject vibration, heat, and cavitation. This causes premature bearing and mechanical seal failures due to shaft deflection, and heat will cause seizure of close tolerance parts and cavitation.

A high efficiency pump uses less energy ($$$) to operate than a low efficiency pump. If possible, it is best to avoid any pump that has an efficiency of 55% or less. (55% efficiency is the industry standard used to estimate the performance of a pump when the actual efficiency is unknown.)

Pump Curve
A pump curve is a simple graph which shows the performance characteristics of a particular pump.

Pump curves are created by the pump manufacturer based on test results of the various pump models the manufacturer produces.

Remember, there is always an inverse relationship between pressure and flow. Higher pressures mean lower flows. Lower pressures result in higher flows.


Each pump curve typically reflects a single model of pump made by the manufacturer. (A typical pump curve is shown at the right)

The top right of the chart shows the pump speed; in the chart above this is 3500 RPM. Two variables affect the pump performance, horsepower of the motor and the size of the impeller.

The left side of the curve is labeled HEAD - FT. This is the distance the pump is capable of lifting the fluid. The bottom of the curve is labeled US GPM. This is the flow that the pump produces.

The red upper curved lines represent the various impeller sizes. The green straight lines represent the motor horsepower ratings available for this pump. Together they represent the best performance the pump is capable of with a selected motor or impeller size.

If a pump is only available with one motor, it will not have separate horsepower lines. If the pump is available with only one size of impeller, there will be just a single line on the entire pump curve!

Based on the above curve, an output of 125 ft hd at 100 GPM would require a 5 HP motor and a 6 inch diameter impeller. Similarly, an output of 70 ft. hd. at 80 GPM would require a 3 HP motor with a 5 inch diameter impeller.

NPSH FT (shown in the lower right of the graph) is the maximum height that a pump can be above the water. This does not apply to submersible, jet, or turbine pump because the pump is underwater. It also does not apply to booster pumps because water is already being forced into them from the water source.

Pumps can sometimes be ordered with custom impeller sizes. This often does not cost much more than a stock pump, but it will delay the delivery since they are custom built.

One of the claimed advantages of the centrifugal pumps over positive displacement pumps is their ability to operate over a wide range of flow. Since a centrifugal pump operates at the intersection of a pump curve and a system curve, by varying the system curve the operating point of the pump is easily changed:

 

Figure 1-1 Flow control of the centrifugal pump by the discharge valve

 

The convenience and simplicity of such flow control by the discharge valve throttling comes at a price, because a pump is thus forced to run either to the left, or to the right, of it's best efficiency point (BEP). However, the real danger of operating the pump too far off-peak comes from the suction side considerations. Too far to the right - and you are easily risking to run out of the available NPSHA, causing cavitation problems. Too far to the left - flow recirculation at the impeller eye will let itself known through the noise, vibration, and damage. Thus, the flow must be limited on both sides of the BEP:

 

Figure 1-2 Pump operating range has limits

 

Consider the first limitation - high flow. Centrifugal pump stops pumping when liquid turns to vapor. This happens when the pressure somewhere inside the pump drops below liquid vapor pressure. Vapor pressure depends on the temperature, and a few other things. As we know, water turns to vapor at 212 ^oF at atmospheric pressure, when we boil water in the open pot. If the pot were closed, the water would reach higher pressure before it boils. Conversely, if the pressure were reduced (vacuum), water would boil at lower temperature. It will boil at room temperature, if the absolute pressure is less then about 0.4 psia. Water has low vapor pressure, but other substances may have very high value.

 

Freon, for example, has vapor pressure of about 90 psia, and ethane value of vapor pressure is about 700 psi, - at 80 ⁰F. Knowing vapor temperature without relating it to a corresponding temperature is meaningless. Sometimes it is good to have a tabulation, or a graph, showing the relationship between the vapor pressure and temperature. The higher the temperature - the higher the vapor pressure is.

 

Centrifugal pump is a "pressure generator", produced by the centrifugal force of its rotation impeller. The pressure gets higher as flow progresses from the suction to discharge. This is why vaporization of liquid is most likely to happen in the inlet (suction) region, where the pressure is lowest. In practice, it is difficult to know exactly when vaporization (cavitation) happens, so it is wise to keep some margin of available pressure over vapor pressure. Pressure is expressed in "psi", but can also be expressed in feet of water, and the conversion formula is:

 

FT = PSI x 2.31 / SG, where SG is specific gravity.

 

This pressure, expressed in feet of water, is called discharge head at the pump exit side, or suction head on the inlet side. The difference is a pump developed head, also called a total dynamic head (TDH). These heads must include both static and dynamic components. Static part is what we measure by the gage in front of a pump, and dynamic, according to Bernoulli, is velocity head V²/2g.

 

For example, suppose an inlet pressure gage installed in a 2" pipe directly in front of a pump delivering 100 gpm oil with specific gravity SG = 0.9, reads 10 psig. To calculate velocity head, find the pipe net area, which is A = 3.14 x d² / 4 = 3.14 x 2² / 4 = 3.1 in².

 

The velocity can be calculated by the formula:

 

V = (Q x 0.321) / A = (100 x 0.321) / 3.1 = 10.4 ft / sec

 

Then, the velocity head is:

 

V² / 2g = 10.4² / (2 x 32.2) = 1.7 ft, or, converted to psi is

 

= 1.7 x 0.9 / 2.31 = 0.7 psi

 

The total suction pressure is then 10 + 0.7 = 10.7 psi, or, if expressed in feet of water,

 

= 10.7 x 2.31 / 0.9 = 27.5 feet

 

It is best to have gages as close as possible to the pump, on the suction and discharge sides. Unfortunately, often these gages are not installed, (which somehow happens more often on the suction side), and suction head in front of the pump is estimated by calculations, by subtracting the pressure (head) losses from the known value of head upstream, and adjusting by elevation correction, according to Bernoulli. In many cases, the upstream datum is a known liquid level in a suction tank.

 

Examples:

 

a) Tank open to atmosphere:

Figure 1-3a: Open tank

 

 

 

Figure 1-3b: Pressurized tank

 

 

h_suction = 2.7x2.31/0.9 + 10 – 7 = 9.9’

 

Figure 1-3c: Tank under vacuum

 

For water and similarly low viscosity liquids, suction losses are usually low, and often are disregarded. However, for more viscous substances, such as oils, these losses can be substantial, and may cause the pressure in front of the pump drop below the vapor pressure, causing cavitation. This is why the inlet velocity must be minimized, as the losses depend on velocity squared.

 

Longer pipe runs, bends, turns and other restrictions, add to inlet losses, leading to further pressure reduction in front of a pump. As a quiz, using the examples above, see if you can figure out what happens to inlet pressure if the pipe diameter is doubled? Or made half the diameter? (If you do – send the answer to us, and will publish it the Pump Magazine).

 

To avoid cavitation, what matters is not the suction pressure, but how much higher it is then the vapor pressure of the liquid being pumped. This is where a concept of NPSH comes handy. The available NPSHA thus is simply the difference between this total suction head, as discussed above, and vapor pressure, expressed as head, in feet.

 

Pump manufacturers conduct tests by gradually lowering suction pressure, and observing when things begin to get out of hands. For a while, as pressure decreases (i.e. NPSHA gets smaller), nothing happens, at least nothing obvious. A pump, operating at a set flow, keeps on pumping, and develops constant head. At some point, when the value of suction pressure (and corresponding NPSHA), reaches a certain value, a pump head begins to drop, which typically happens rather suddenly:

Figure 1-4: Development of Cavitation

 

Actually, things are happening inside the pump well before the sudden drop of head, but they are not as obvious. First, at still substantial suction pressure, small bubbles begin to form. This is called incipient cavitation - sort of tiny bubbles in your water cattle that begins to percolate before water is fully boiling. These small bubbles are formed and collapse, at very high frequency, and can only be detected by the special instrumentation. As pressure is decreased further, more bubbles are formed, and eventually there are so many of them, that the pump inlet becomes "vapor-locked", so that no fluid goes through, and the pump stops pumping - the head drops and disappears quickly. It would be nice if enough pressure was always available at the suction so that no bubbles were formed whatsoever. However, this is not practical, and some compromise must be reached. The Hydraulic Institute (HI) has established a special significance to a particular value of NPSHA, at which the pump total developed head drops by 3%. The value of this NPSHA, at which a pump losses 3% TDH, over (i.e. in access of) vapor pressure is called net positive suction head required (NPSHr) in order to maintain 3% TDH loss.

 

NPSHr = (H_suction - H_vapor), required to maintain 3% TDH loss

 

NPSHr is, therefore, established by actual test, and may vary from one pump design to another.

 

In contrast, the available NPSHa, has nothing to do with a pump, but is strictly a calculated value of total suction head over vapor pressure. Clearly, NPSHA must be greater then NPSHR, in order for a pump to make its performance, i.e. to deliver a TDH, at a given flow.

 

It is easy to know when a NPSH problem is obvious - a pump just stops pumping, but the vapor bubbles do not need to be so dramatically developed to cause TDH drop, - even smaller bubbles can cause problems. The evolved bubbles get carried on through the impeller passage, at which pressure is rising from inlet to exit of the blade cascade. This increased pressure causes the reverse to what happened to a bubble "awhile back", when it first became a bubble formed from a liquid particle during phase transformation (boiling). Now, the bubble is at the somewhat higher pressure, which tries to squeeze it, against the vapor surface tension that keeps the bubble a bubble. The bubble collapses (implodes), with a sudden in-rush of surrounding liquid into a vacuum space previously occupied by the bubble. The inrush is accompanied by a tremendous, but a very localized, pressure shock, which, if imploded in the vicinity of the metal (impeller blade), would cause a microscopic hammer-like impact, eroding a small particle of metal. With enough bubbles and enough time, the impeller vanes can be eroded away quickly, a phenomenon known as cavitation (hence the word) damage.

 

This is why an NPSHA margin (M=NPSHA-NPSHR) is important, which is typically at least 3-5 feet, and preferably should be even more, if possible.

 

The NPSHR, discussed above, was so far limited to a particular flow on a pump performance curve. At higher flow, the internal fluid velocities are higher, and, according to Bernoulli, the static pressure (or static head) part becomes less, i.e. closer to vapor pressure. The static pressure, therefore, must be increased externally, i.e. a higher value of NPSHR is needed for higher flows. This is why the NPSHR curve shape looks like this:

Figure 1-5: Ample margin of NPSHA is important

 

It is important to specify an ample margin of NPSHA over the pump NPSHR for a complete range of operation, and not just at a single rated flow point. The following example illustrates a common mistake, leading to the NPSH-problem. The pump was procured with the intend to deliver between 350-500 gpm, and the manufacturer quotation indicated 16 feet required NPSHR at 500 gpm. As a process later changed, more flow was required, and the discharge valve was opened to allow pump to deliver more flow, 750 gpm. However, as can be seen from Figure 1-5, at about 700 gpm, the NPSHR exceeded the NPSHA available at the installation, and pump started to experience typical NPSH problems - noise, loss of performance, and impeller cavitation damage.

 

An instinctive thought to address the issue of cavitation due to flow-run out operation is to "overkill" on a pump size, and therefore always stay to the left of the BEP. In the example above, a larger pump, having same 16 feet NPSHR, but at 750 - 800 gpm, would never run out of the NPSHA. That is true, and, in fact, this is exactly what has been a common practice in the past, where an oversized (and, by the way, more expensive) pump would be specified "to make sure", - just to discover other, just as severe problems.

 

When a centrifugal pump operates below certain flow point, a phenomenon known as flow recirculation in the impeller eye starts. This depends on several design factors, such as suction specific speed (see in other article of Pump Magazine), but generally recirculation begins below 80-60% flow, and becomes quite sever below 40-20%. At even lower flows, recirculation may become especially severe, and is known as surge - violent, low-frequency sound, accompanied by strong low-frequency vibration of the pump and piping:

 

Figure 1-6 Problems come up when pump operates at too low flow

 

In addition to obvious mechanical problems with recirculation, the flow undergoes a complex vortexing motion at the impeller inlet (eye), with localized high velocities of the vortex causing horse-shoe looking cavitation damage, usually on the "blind" side of the blade, as compared to high-flow cavitation. Other problems add oil to the fire - radial thrust, which sky-rockets at low flow, causes deflections of the shaft, leading to seal leaks, bearings life reduction, and even shaft breakage (see other articles of the Pump Magazine on these subjects).

 

Troubleshooting methods and failure analysis techniques help to pinpoint a cavitation problem with a particular pump. The indications of the high flow cavitation are different from the low flow recirculation damage. Side of the blades, the extend and shape of the cavitation trough, can be helpful in determining the causes of each individual problem.