Ways to Save Energy in Pumps

Here is my experience based on energy audits of pumping systems in various chemical, metal, textile & petrochemical units.

• Design systems with lower capacity and total head

1. Do not assume these requirements are fixed.
2. Calculate flow requirement based on actual mathematical nos without margins in each stage & then add 10-20% straightforward as Normal capacity of the pump. For example if process side heat load in an exchanger is based on normal flow of say 100 M3/hr then do not consider cooling water requirement for peak condition of 120 or say 140 M3/hr. Just calculate it based on normal flow of 100 M3/hr at this stage.
3. Total head requirements can be reduced by: lowering process static gage, pressure, minimizing elevation rise from suction tank to discharge tank, reducing static elevation change by use of siphons, lowering spray nozzle velocities, lowering friction losses through use of larger pipes and low-loss fittings, and eliminating throttle valves.
4. After calculating total requirement of Flow & Head this way, simply add 10-20% in both parameters as design margin based on your judgement about process variation. This should be your normal capacity. You still have higher margins becasue rated condition are further higher than these values.
5. Also keep in mind that worst conditions dont come all simultaneously. You can still meet few peak demands.
6. Dont worry about undersizing of your pump. You can add later & this approach is beneficial in overall longer run as you can switch your additional capacity ON & OFF.
7. This can give you a saving of ~10-20% in your pumping system. A thorough review is must.

• Emphasize on Efficiency first

1. Despite the tendency to emphasize initial cost, you will save cost in the long run by selecting the most efficient pump type and size at the onset.
2. The choice of a pump depends on the service needed from the pump. Considerations are flow and head requirements, inlet pressure or net positive suction head available, and the type of liquid to be pumped.
3. Maximum attainable efficiency of a centrifugal pump is influenced by the designer's selection of pump rotating speed as it relates to "specific speed." Purchasers need to be aware of this, as well as the decision criteria for determining the type of pump to use.
4. Consider LCC (Life Cycle Costing) option instead of initial cost only. Click here to learn more about LCC Analysis.
5. Remember ENERGY is the most expensive "commodity" today.
6. People generally loose ~80% more money due to non LCC approachover a period of its service life.

• Divided Use

1. Design or select no of pumps based on different possible scenarios & always follow the operation philosophy of bulk & makeup supply for any system.
2. This approach saves at least 10-15% over conventional selection of equal size pumps.
3. This helps in putting the smaller pump on auto mode with header pressure switch so that excess pump capacity can be turned on/off.
4. Two pumps can be operated in parallel during peak demand periods, with one pump operating by itself during lower demand periods. Energy savings result from running each pump at a more efficient operating point and avoiding the need to throttle a large pump during low demand.
5. Analternative is to use one variable-speed pump and one constant-speed pump. Use or selection depends on the process behaviour e.g. how fast the demand is changing? How many time it is changing? Is my process critical? etc.

• Avoid end of curve operation

1. Generally in case of cooling water pumps head & flow both are selected with plenty of margins, comfortable to the cushion needed by selection manager. This results in near end curve operation without throttling. This is worst operation of pumps in almost every situation. Avoid it.

• Use pumps as drives

1. Use them as drivers / turbines to recover pressure energy that would otherwise be wasted.
2. Practically all centrifugal pumps will perform as turbines when operated in reverse.
3. A hydraulic power recovery turbine can recover pressure energy when used to drive a generator, or assist the driver of a pump or a compressor.

Useful Tips on Use of ASD/VFD for pumps

ASDs are ideally suited for variable-torque loads from centrifugal pumps, fans, and blowers when the system load requirements (head, flow, or both) vary with time. Conditions that tend to make ASDs cost-effective include the following:

1. High horsepower (greater than 15 to 30 hp)—the higher the pump horsepower, the more cost-effective the ASD application.
2. Load type—Centrifugal loads with variable-torque requirements (such as centrifugal pumps or fans) have the greatest potential for energy savings. ASDs can be cost-effective on positive displacement pumps, but the savings will generally not be as great as with centrifugal loads.
3. Operating hours—In general, ASDs are cost-effective only on pumps that operate for at least 2,000 hours per year at average utility rates.
4. High utility rates—higher utility energy charges provide a more rapid payback on an investment in an ASD.
5. Availability of efficiency incentives—where they are available, electric utility incentives for reducing energy use or installing energy-saving technologies will reduce payback periods.
6. Low static head—ASDs are ideal for circulating pumping systems in which the system curve is defined by dynamic or friction head losses. They can also be effective in static-dominated systems—but only when the pump is carefully selected. A thorough understanding of pump and system interactions is critical for such applications.
   
   
   
   
   Courtesy of  Profmaster

Pump Efficiency

Multiply the pump head by the pump's capacity, and then use a simple conversion number.

Flow = 300 gallons per minute of fresh water as measured coming from the pump discharge.
Head = 160 feet. We measured it at the discharge side of the pump and corrected it for the fact that the gage was two feet above the pump center line.
Note:If there were any positive head on the suction side of the pump that head would have to be subtracted. A negative suction head would be added to the discharge head.

 The centrifugal pump pumps the difference between the suction and the discharge heads. There are three kinds of discharge head:

Static head. The height we are pumping to, or the height to the discharge piping outlet that is filling the tank from the top. Note: that if you are filling the tank from the bottom, the static head will be constantly changing.

Pressure head. If we are pumping to a pressurized vessel (like a boiler) we must convert the pressure units (psi. or Kg.) to head units (feet or meters).

System or dynamic head. Caused by friction in the pipes, fittings, and system components. We get this number by making the calculations from published charts

Suction head is measured the same way.

If the liquid level is above the pump center line, that level is a positive suction head. If the pump is lifting a liquid level from below its center line, it is a negative suction head.
If the pump is pumping liquid from a pressurized vessel, you must convert this pressure to a positive suction head. A vacuum in the tank would be converted to a negative suction head.
Friction in the pipes, fittings, and associated hardware is a negative suction head.
Negative suction heads are added to the pump discharge head, positive suctions heads are subtracted from the pump discharge head.

Here is the formula for measuring the horsepower out of the pump:

 

Remember that we are using the actual horsepower or kilowatts going into the pump and not the horsepower or kilowatts required by the electric motor. Most motors run some where near 85% efficient.

An 85% efficient motor turning a 76% efficient pump, gives you a real efficiency of 0 .85 x 0.76 = 0.65 or 65% efficient.

A survey of popular pump brands demonstrates that pump efficiencies range from 15% to over 90%. The question then arises, "Is this very wide range due to poor selection, poor design, or some other variable which would interfere with good performance?" The best available evidence suggests that pump efficiency is directly related to " the specific speed number " with efficiencies dropping dramatically below a number of 1000 . Testing also shows that smaller capacity pumps exhibit lower efficiencies than higher capacity designs.

Ref: mcnallyinstitute 

Cavitation in Pumps

Cavitation means that cavities or bubbles are forming in the liquid that we're pumping. These cavities form at the low pressure or suction side of the pump, causing several things to happen all at once:
The cavities or bubbles will collapse when they pass into the higher regions of pressure, causing noise, vibration, and damage to many of the components.
We experience a loss in capacity.
The pump can no longer build the same head (pressure)
The pump's efficiency drops.

The cavities form for five basic reasons and it's common practice to lump all of them into the general classification of cavitation. This is an error because we'll learn that to correct each of these conditions, we must understand why they occur and how to fix them. Here they are in no particular order :
Vaporization
Air ingestion (Not really cavitation, but has similar symptoms)
Internal recirculation
Flow turbulence
The Vane Passing Syndrome

Vaporization .

A fluid vaporizes when its pressure becomes too low, or its temperature too high. All centrifugal pumps have a required head (pressure) at the suction side of the pump to prevent this vaporization. This head requirement is supplied to us by the pump manufacturer and is calculated with the assumption that fresh water at 68 degrees Fahrenheit (Twenty degrees Centigrade) is the fluid being pumped.

Since there are losses in the piping leading from the source to the suction of the pump, we must determine the head after these losses are calculated. Another way to say this is that a Net Positive Suction Head is Required (N.P.S.H.R.) to prevent the fluid from vaporizing.

We take the Net Positive Suction Head Available (N.P.S.H.A.) subtract the Vapor Pressure of the product we are pumping, and this number must be equal to or greater than the Net Positive Suction Head Required.

To cure vaporization problems you must either increase the suction head, lower the fluid temperature, or decrease the N.P.S.H. Required. We shall look at each possibility:

Increase the suction head
Raise the liquid level in the tank
Raise the tank
Pressurize the tank
Place the pump in a pit
Reduce the piping losses. These losses occur for a variety of reasons that include :
The system was designed incorrectly. There are too many fittings and/or the piping is too small in diameter.
A pipe liner has collapsed.
Solids have built up on the inside of the pipe.
The suction pipe collapsed when it was run over by a heavy vehicle.
A suction strainer is clogged.
Be sure the tank vent is open and not obstructed. Vents can freeze in cold weather
Something is stuck in the pipe, It either formed there, or was left during the last time the system was opened . Maybe a check valve is broken and the seat is stuck in the pipe.
The inside of the pipe, or a fitting has corroded.
A bigger pump has been installed and the existing system has too much loss for the increased capacity.
A globe valve was used to replace a gate valve.
A heating jacket has frozen and collapsed the pipe.
A gasket is protruding into the piping.
The pump speed has increased.
Install a booster pump

Lower the pumping fluid temperature
Injecting a small amount of cooler fluid at the suction is often practical.
Insulate the piping from the sun's rays.
Be careful of discharge recirculation lines. They can heat the suction fluid.

Reduce the N.P.S.H. Required
Use a double suction pump. This can reduce the N.P.S.H.R. by as much as 25%, or in some cases it will allow you to raise the pump speed by 40%
Use a slower speed pump.
Use a pump with a larger, impeller eye opening.
If possible, install an Inducer. These inducers can cut N.P.S.H.R. by almost 50%.
Use several smaller pumps. Three half capacity pumps can be cheaper than one large pump plus a spare. This will also conserve energy at lighter loads.

It's a general rule of thumb that hot water and gas free hydrocarbons can use up to 50% of normal cold water N.P.S.H. requirements, or 10 feet (3 meters), whichever is smaller. I would suggest you use this as a safety margin, rather than design for it.

Air ingestion (Not really cavitation, but acts like it)

A centrifugal pump can handle 0.5% air by volume. At 6% air the results can be disastrous. Air gets into as system in several ways that include :
Through the packing stuffing box. This occurs in any packed pump that lifts liquid, pumps from a condenser, evaporator, or any piece of equipment that runs in vacuum.
Valves located above the water line.
Through leaking flanges.
Pulling air through a vortexing fluid.
If a bypass line has been installed too close to the suction, it will increase the temperature of the incoming fluid.
Any time the suction inlet pipe looses fluid. This can occur when the level gets too low, or there is a false reading on the gauge because the float is stuck on a corroded rod.

Both vaporization and air ingestion have an adverse affect on the pump. The bubbles collapse as they pass from the eye of the pump to the higher pressure side of the impeller. Air ingestion seldom causes damage to the impeller or casing. The main effect of air ingestion is loss of capacity.

Although air ingestion and vaporization can both occur, they have separate solutions. Air ingestion is not as severe as vaporization and seldom causes damage, but it does lower the capacity of the pump.

Internal Recirculation

This condition is visible on the leading edge of the impeller, close to the outside diameter, working its way back to the middle of the vane. It can also be found at the suction eye of the pump.

As the name implies, the fluid recirculates increasing its velocity until it vaporizes and then collapses in the surrounding higher pressure. This has always been a problem with low NPSH pumps and the term Suction Specific Speed to guide you in determining how close you have to operate to the B.E.P. of a pump to prevent the problem.

The higher the number the smaller the window in which you can operate. The numbers range between 3,000 and 20,000. Water pumps should stay between 3,000 and 12,000. Here is the formula to determine the suction specific speed number of your pump:


rpm = Pump speed

Capacity = Gallons per minute, or liters per second of the largest impeller at its BEP

Head= Net positive suction head required (feet or meters) at that rpm
For a double suction pump the flow is divided by 2 since there are 2 impeller eyes
Try to buy pumps with a suction specific speed number lower than 8500.(5200 metric ) forget those over 12000 (8000 metric) except for extreme circumstances.
Mixed hydrocarbons and hot water at 9000 to 12000 (5500 to 7300 metric) or higher, can probably operate satisfactorily.
High specific speed indicates the impeller eye is larger than normal, and efficiency may be compromised to obtain a low NPSH required.
Higher values of specific speed may require special designs, and operate with some cavitation.
Normally a pump operating 50% below its best efficiency point (B.E.P.) is less reliable.

With an open impeller pump you can usually correct the internal recirculation problem by adjusting the impeller clearance to the manufacturers specifications. Closed impeller pumps present a bigger problem and the most practical solution seems to be to contact the manufacturer for an evaluation of the impeller design and a possible change in the design of the impeller or the wear ring clearances.

Turbulence

We always prefer to have liquid flowing through the piping at a constant velocity. Corrosion or obstructions can change the velocity of this liquid, and any time you change the velocity of a liquid, you change its pressure. Good piping layouts would include :
Ten diameters of pipe between the pump suction and the first elbow.
In multiple pump arrangements locate the suction bells in separate bays so that one pump suction will not interfere with another. If this is not practical, a number of units can be installed in a single large sump provided that :
The pumps should be positioned in a line perpendicular to the approaching flow.
There must be a minimum spacing of at least two suction diameters between pump center lines.
All pumps are running.
The upstream conditions should have a minimum straight run of ten pipe diameters to provide uniform flow to the suction bells.
Each pump capacity must be less than 15,000 gpm..
Back wall clearance distance to the centerline of the pump must be at least 0.75 of the suction diameter.
Bottom clearance should be approximately 0.30 (30%) of the suction diameter
The minimum submergence should be as follows:

FLOW MINIMUM SUBMERGENCE

20,000 GPM
4 FEET

100,000 GPM
8 FEET

180,000 GPM
10 FEET

200,000 GPM
11 FEET

250,000 GPM
12 FEET

The metric numbers are :
FLOW MINIMUM SUBMERGENCE

4,500 M3/HR
1.2 METERS

22,500 M3/HR
2.5 METERS

40,000 M3/HR
3.0 METERS

45,000 M3/HR
3.4 METERS

55,000 M3/HR
3.7 METERS


The Vane Passing Syndrome

This type of cavitation damage is caused when the OD of the impeller passes too close to the pump cutwater. The velocity of the liquid increases as it flows through this small passage, lowering the fluid pressure and causing local vaporization. The bubbles then collapse at the higher pressure just beyond the cutwater. This is where you should look for volute damage. You'll need a flashlight and mirror to see the damage, unless it has penetrated to the outside of the volute.

The damage is limited to the center of the impeller vane. If it's a closed impeller, the damage will not extend into the shrouds. You can prevent this problem, if you keep a minimum impeller tip to cutwater clearance of 4% of the impeller diameter in the smaller impeller sizes (less than 14' or 355 mm.) and a 6% clearance in the larger impeller sizes (greater than 14" or 355 mm.).

To prevent excessive shaft movement, some manufacturers install bulkhead rings in the suction eye. At the discharge side, rings can be manufactured to extend from the walls to the impeller shrouds.

(Ref: mcnallyinstitute)