Simple Way to Determine Suction Lift

Note: Cornell Pump Market Managers provide periodic articles to the blog, to discuss issues and developments and pump. The articles are meant to be more conversation and less technical, while still explaining important pumping concepts. In this edition, Cornell Pump Industrial Market Manager Derek Petersen discusses a method to determine suction lift. 

People always ask how much of a suction lift will a Cornell Pump pull?   The answer is simple and easy to calculate in four steps:

1. Determine the NPSHR (provided on pump curve)
2. Look up the amount of pressure the atmosphere (weight of air) is pushing down at a particular elevation.
3. Calculate how much energy all the valves, pipe, fittings, elbows, etc., in the pump system cost the system in terms of flow.
4. Compare how much the system could produce versus how much energy it going to cost.

The first thing to understand is that every pump has an energy requirement needed to run without cavitation.  This required energy is call the Net Positive Suction Head Required or NPSHR.  Thinking of NPSHR another way, it’s the absolute pressure a liquid must have to avoid creating microscopic, damaging vapor bubbles in the liquid being pumped. Those bubbles are cavitation and they can harm a pump and shorten its useable life.

The NPSHR is inherently part of the pump design [how steep the impeller vanes, the speed of operation, the shape of the volute, etc.,] and is listed on the pump curve at your specific design point.  NPSHR is calculated by Cornell Pump in our test lab empirically. It is important to remember that the NPSHR will vary at different operating conditions for a pump, and can be different for the same operating conditions when comparing two different pumps.

Next, the maximum a centrifugal pump can pull is constrained by nature. Atmospheric pressure exerts about 14.7 pounds per square inch of force on everything (you, a car, liquid) at sea level. That 14.7 psi on liquid allows it a maximum of 34 feet of head (push) at sea level. Again these values have been calculated for you. Not by pump manufacturers per se, but starting with enlightenment scientists looking to understand barometric and atmospheric pressure. Atmospheric pressure is understood today to be about 34 feet of head at sea level. If you were on the top of Mount Everest the psi would only be 4.4 and the energy impart would be 10.2 feet of head as a maximum.

So, you have a known amount in NPSHR provided by the pump manufacturer for the particular model, and you can consult a chart on atmospheric pressure.

In the third step, you have to do some leg work and round up everything (pipes, valves, etc.) that the liquid will travel through.  Each of these items are not completely smooth, and in the case of elbows etc. are not straight either. Liquid moving through the parts will lose some energy running over the less than smooth bumps that exist (like a stream running into a rock—it gives up some energy and creates an eddy behind the rock.)

You will need to subtract all other losses from the equipment in order to determine the NPSHA of your system.  Losses would also include your static suction lift in feet or the vertical distance from the water level to pump. The friction loss in feet in the suction pipe or the pressure lost when the water rubs against the walls of the pipe and losses created from vapor pressure which is a result of the temperature of the liquid.

It also advisable to include a safety factor to NPSHA. In case of storms (causing lower atmospheric pressure), changes in pipe diameter due to corrosion, etc., the factor allows the pump to operate in less than ideal conditions. Cornell recommends adding 2’ of loss for the NPSH margin.

Finally, once you know the losses for your NPSHA, you can add that to NPSHR, and then compare it the atmospheric pressure for the elevation.  That will be the amount of static lift available.

Note: In order to avoid cavitation and pull a suction lift with your pump the NPSHA of your system must always be higher than the NPSHR (of the Pump).

The equation for determining how much of a suction lift you can pull with your pump you can take your Atmospheric pressure(Pb) subtract your Pump NPSHR, Vapor Pressure (Vp), friction losses (hf) and NPSH Margin (Safety factor) and you will have your maximum suction lift.

• Suction Lift = P(b) – (Ls + Vp + hf + NPSHR)
• P = Pressure (in ft) at surface of water
• Pb = Barometric Pressure (open system)
• P   = Absolute Pressure at surface of liquid   (closed system) 
• Ls & Lh = Distance from water level to pump CL
• Ls is below pump centerline
• Lh is above pump centerline
• hf = Friction Losses in Suction Pipe
• Vp = Vapor Pressure of liquid.
• NPSHR (from pump curve)

Here is an example.

Friction losses in pipe, vapor pressure at elevation and Atmospheric pressure at different elevations are available in the Cameron Hydraulic Data Book.

Packing vs Mechanical Seal

Note: Cornell Pump Market Managers provide periodic articles to the blog, to discuss issues and developments and pump. The articles are meant to be more conversation and less technical, while still explaining important pumping concepts. In this edition, Cornell Pump Agricultural Market Manager Bob Jansen discusses differences between packing and mechanical seals.

Packing is the traditional method of stopping leakage around the drive shaft of an end-suction centrifugal pump. Rings of braided, fibrous material, such as graphited or non-graphited acrylic, PTFE (polytetrafluoroethylene or Teflon) or other materials, are “stuffed” into a pump stuffing box (or seal chamber), located in the pump backplate, around the outside diameter of the pump shaft, to reduce the high pressure developed in the pump case, and decrease the amount of pumped medium that is forced out of the pump along the drive shaft. Packing does not stop all leaking, however. Packing rings are kept just loose enough to allow a trickle of liquid to seep out during operation, which accomplishes the flushing action necessary to prevent overheating and excessive wear between the inside of the packing surface and the outside diameter of the drive shaft. 

Advantages of using packing normally include lower upfront cost, availability, simple installation and is replaceable with minimum down time. The disadvantages of packing are that it needs to be monitored and adjusted, as required, to maintain a slow drip (40-60 drops/minute) to cool and lubricate the area. In addition, the area surrounding the pump needs to be able to handle the small amount of constant and necessary leakage from the packed stuffing box – such as removing the liquid from the area, etc.

Packing is available from most manufacturers in pre-formed sets for each individual model, or in bulk form. Various materials are available to match the requirements of nearly every application.

Mechanical seals are another common method to seal this area of the pump. They consist, in their most basic form, of two flat faces (machined within light bands tolerance): a rotating element that spins with and seals the pump shaft, and a static element that presses into the backplate casting and seals the liquid from escaping to the atmosphere. The central advantage that mechanical seals possess over traditional packing is that they almost completely negate all forms of leaking in a pump. (A small amount of liquid vaporizes as it crosses the seal face, but is usually not noticeable). Less maintenance time is required with a mechanical seal during operation, as packing must be regularly inspected and adjusted/replaced, whereas a mechanical seal operates, without attention, until the seal faces are badly worn. Mechanical seals come in a number of possible configurations and materials for different pump types, medium pumped, pressure ranges required, etc., but for this post we will only mention the two overarching categories of single and double seals.

As with all mechanical seals, they require careful and informed installation, but once properly set in place, they require no additional adjustment or maintenance. Mechanical seals often present more upfront cost than packing – sometimes considerably more. However, because mechanical seals usually offer less downtime and maintenance than packing, they can save money over the life of the pump. Seals are also less tolerant to shaft deflection and misalignment, and dirty or contaminated medium.

Since mechanical seals require a thin liquid film to properly lubricate and cool the faces, especially with abrasive or corrosive pump mediums, the seal faces will be eroded over time. Proper materials must be selected to be compatible with the pumping medium, and can be expensive.

For hazardous or more severe pumpage, a double (dual pressurized) seal may be necessary. In this system, there is essentially no leakage allowed outside the pump. Double seals are far more durable than single seals and may last up to five times longer in this environment. A flushing liquid is required for double seal installations.

One method Cornell developed to assist with solids and abrasive applications is our patented Cycloseal® technology. Cornell’s “Run-Dry” System lubricates the mechanical seal and allows the pump to run without pumping liquid. Contact the factory for additional info on these sealing technologies.

Try Quiz #2 –Test your Pump Knowledge

How well do you know pumps? Take the second in our quiz series to test your general centrifugal pump knowledge!
Cornell Pump will be holding our annual pump school in January 2017. We normally hold the school in September, but this year we’re accommodating those who aren’t able to attend because of harvest, etc.
The quiz is short—five questions long—and you get the answers immediately. Take the quiz to brush up on your knowledge, or confirm your ability to get five out of five correct! All these topics and much more will be discussed at Pump School 2017.

Take the Quiz Now

You can learn about pump school and download the registration form on the Cornell website. Pump School is a great value; $175 per attendee, or ONLY $95 if you come as a guest of a Cornell Distributor. Three days, two receptions, meals, giveaways, and TCH-approved learning in one of the most beautiful cities in the country. Make your plans to attend Pump School 2017, January 23-25, 2017; Monday night reception and classes/factory training on Tuesday and Wednesday.

Do you know how to successfully start-up a pump?

Cornell Pump sells thousands of pumps each year—and every one of them has to put into service for the first time in the field or at the installation.  Pumps also need to checked if they have not been used for a period of time, for instance, after the last growing season.

To aid operators, we go over start-up procedures at our pump school. Following is a checklist of activities, in order of action, that we have found will help a start-up go smoothly.

Whether you’ve never started up a pump before, or you’re an old-hand at hydraulics, this 18-point check list can help ensure your next pump start-up is trouble free. And, if you’d like more training about start-up and other operation/maintenance related topics, consider attending the Cornell Pump 2017 Pump School. It will be held January 24 and 25, 2017 in Portland, Oregon. Get more information about Pump School 2017.

START-UP CHECK LIST

1. Re-read all instructions and check for compliance on each point.
2. Piping must be clean and free of debris and obstructions, gaskets in place and all joints secure.
3. Are all thrust blocks and supports adequate?
4. Are screens in place?
5. Check the valves and blow-offs for proper position.
6. Make sure support systems are in place and functioning, such as special lubrication, frame oil, etc.
7. Check the power supply voltage with the motor name plate.
8. Are belts and shaft couplings properly adjusted and aligned and guards in place?
9. Does the pump rotate freely?
10. Prime the pump.
11. Check pump rotational direction.  (VERY SHORT on/off power pulse).
12. Comply with all seal or packing operation and start-up instructions.
13. Monitor the motor temperature.
14. Note the operating temperature of frame bearings (if any).
15. The pump may be checked for shut-off pressure with the pump performance curve.
16. Fill the system slowly.
17. Do not operate any pump without properly priming it, unless it has been specifically designed for such operation.
18. New pumps must not be started and stopped frequently.  If possible, permit the unit to run until operating temperature is reached.

NOTE: Large motors must not be started and stopped more than five times per hour.

A pump must not be started until compliance is reached on all the applicable points above and any others specified in the “Operation and Maintenance Manual” supplied with the pump. Failure to do so may cause severe damage to equipment and/or personal injury. It may also void the warranty.

This is a first in a series about pump-start up considerations. Look for additional articles over the next several days.