Getting In Your Pump’s Head

Larry Bachus

It is a fact that too many industrial pumps are problematic, and ill suited for the application. Many people are aware that the water pump on the radiator of their car will run 12 to 15 years, and the Freon compressor on their refrigerators will last 18 to 20 years without problems. In light of such pumping dependability, plant operators are concerned that they can’t get similar life out of their circulating chill water and cooling tower pumps. Too many industrial pumps seem to eat bearings and seals and spend too much time in the maintenance bay.

Maintenance costs on industrial pumps can get out of control when the pump is inadequately mated to its system. Because the pump must respond to the needs of the system, the root cause of pump failure normally lies in the system.

Figure 1. There are four elements that go into the total head of a pump system: static, pressure, friction, and velocity.

A pumping system has a flow requirement and a head requirement. Flow is production (marketing, sales, consumption). Head is the nature and design of the process of production. For example, if a system requires 200 gallons per minute at 38 feet of head then you’ll need a pump with a best efficiency point (BEP) that coincides with 200 GPM at 38 feet of head on the pump curve, or very close to those coordinates.

It is relatively easy to understand 200 GPM. The 38 feet of system head requires some thought. The 38 feet of system head is called the Total Head (TH). The purists use the term TDH, meaning Total Dynamic Head. You may also hear Total Developed Head and Total Discharge Head. The Total Head, or TDH, is composed of four elements:
1. The elevation change, called the Static Head or Hs.
2. The pressure change, called the Pressure Head, or Hp.
3. The friction losses across the system, called Friction Head, or Hf.
4. The velocity losses across the system, called Velocity Head or Hv.
TDH = Hs + Hp + Hf + Hv (Figure 1). Let’s consider each of these elements.

Hs is static head. If you’re pumping from down here to up there, that distance in feet is the Hs. For example, a pump is used to circulate water in a cooling tower, raising the water from the reservoir pan below the tower to the top of the cooling tower.

If the elevation change is 30 feet, then the Hs = 30 feet. Here is another example. Parts of New Orleans are below sea level and the Mississippi River. Rainwater and storm water must be lifted up over the levies and into the river with pumps. If
this vertical distance is 12 feet, then the pumps must generate 12 feet Hs. By observing the system and noting the elevation change, this distance in feet is the Hs.

Pressure head, Hp, is also rather easy to understand. Hp is the pressure change across the system. If both suction and discharge vessels are exposed to the same pressure (whether high pressure, atmospheric pressure, or vacuum), then there is no differential and no Hp to consider. If there is a pressure differential across the system, then there is Hp. We are generally concerned with positive pressure differential. However, it may be negative if the pressure on the suction vessel or source is higher than the pressure on the discharge vessel.

The calculation is Hp = PSI differential x 2.31/specific gravity. The 2.31 and specific gravity were discussed in a previous Pump Guy column (January , page 26). The pressure differential can usually be observed on the gauges at the suction and discharge vessels.

The friction losses, called Hf, refer to the friction between the liquid and the internal surfaces of the pipes, valves, and fittings. The friction in the pipes reduces or drags down the ability of the pump. The friction losses must be designed into the pump so that it will overcome the frictional resistance of the system. The mathematical formulas are simple. For pipes Hf = K x L/100 where K is a friction constant per 100 feet of pipe derived from pipe friction tables. L is the length of pipe in the system (in feet). For valves and fittings, the formula is Hf = K x Hv, where K is a resistance constant derived from charts and Hv is the velocity head in feet, either calculated or derived from charts.

Finally, we have the velocity losses called Hv. Notice that Hv is a component of friction (see the previous paragraph). Hf and Hv work in concert when the fluid is advancing in a pipe. Creating and maintaining a fluid velocity in the pipes also consumes additional energy that must be designed into the pump. The formula is simple. Hv = V²/2g, or velocity in feet/sec. squared, divided by two-times the acceleration of gravity (32.16 feet/sec.). Because the acceleration of gravity is a constant, then the Hv rises and falls with the fluid velocity.

Most design engineers will specify a pipe based on a fluid velocity of about five to eight feet per second. Slower is better unless there is an engineering reason for high fluid velocity in a pipe.

Once again, TDH = Hs + Hp + Hf + Hv. Let’s consider a system where we must pump and raise ambient water at 200 GPM from one vented tank, up 30 feet into another vented tank. The Hs is 30 feet. The piping, valves, fittings, and a strainer compute to 7.5 feet of friction head. The Hf is 7.5. The water is moving through the discharge pipe at 5.07 feet per sec. The Hv is 0.4 feet. Because both tanks are vented at atmospheric pressure, there is no pressure differential and the Hp is zero. Then we need a pump with a TDH of 38 feet (30 + 0 + 7.5 + 0.4). We should select a pump with a best efficiency point as close as possible to 200 GPM at 38 feet of head.

In almost all cases, you should purchase the pump with the highest efficiency, even if it costs more. Efficiency is tied to the monthly energy costs and maintenance costs. We’ll talk about these in a future
article.

Mating a pump to a system is really quite simple. When it is done correctly, the pump will run for a number of years without problems. Remember that I opened this column talking about your car’s radiator water pump and your refrigerator compressor. The reality of the industrial plant is different. As a pump consultant, I often go into plants whose pump maintenance costs have gone way beyond the budget. I see pumps that consume three and four sets of bearings and seals per year and spend too much time in the maintenance bay. So, what’s the problem with so many industrial pumps?

We’ll talk about it next month. Keep this article or store this magazine where you can find it quickly. The Pump Guy column is your cheatsheet of useful pump information.

Larry Bachus, founder of pump services firm Bachus Company Inc., is a regular contributor to Flow Control magazine. He is a pump consultant, lecturer, and inventor based in Nashville, Tenn. Mr. Bachus is a member of ASME and lectures in both English and Spanish. He can be reached at [email protected] or
615 361-7295.

For More Information: www.bachusinc.com

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