Pressure measurement

Pressure exists in our daily lives. At sea level the atmospheric pressure is 14.7 psia. This is the pressure exerted on us by the air we breathe. If we should remove all the air, then the pressure would be zero.

We're more concerned with pressures above atmospheric pressure. For example, a flat tire on a car still has 14.7 pounds of pressure inside it. We would consider this to be a flat tire because the pressure outside the tire is equal to the pressure inside the tire. We would say the tire has no pressure because it would not be inflated and could not support the weight of the car.

What is more important to us is the differential pressure inside the tire compared to outside the tire (atmospheric pressure). For reasons such as these, the world has adopted a second and artificial zero, at atmospheric pressure as a reference point. This is why a simple pressure gauge will read zero at atmospheric pressure.

Because simple pressure gauges are made with an artificial zero at atmospheric pressure, this is why the term psig exists, meaning pounds per square inch gauge. As mentioned earlier, the psig is equal to the absolute pressure minus the atmospheric pressure.

Pressures less than atmospheric are recorded as negative pressures (—psi) on a simple pressure gauge.

Technically speaking, negative pressures don't exist. Pressure is only a positive force and it is either present or absent.

Pressures inside the pump Suction pressure

Suction pressure is the pressure at the pump's suction nozzle as measured on a gauge. The suction pressure is probably the most important pressure inside the pump. All the pump's production is based on the suction pressure. The pump takes suction pressure and converts it into discharge pressure. If the suction pressure is inadequate, it leads to cavitation. Because of this, all pumps need a gauge at the suction nozzle to measure the pressure entering the pump.

Discharge pressure

This is the pressure at the pump discharge nozzle as measured by a gauge. It is equal to the suction pressure plus the total pressure developed by the pump.

Seal chamber pressure

This is the pressure measured in the stuffing box or seal chamber. This is the pressure to be sealed by the mechanical seal or packing. The seal chamber pressure must be within the limits of the mechanical seal. This

pressure is very important with double mechanical seals, because it governs the pressure setting of the barrier fluid.

Head versus pressure

Figures 1-4: and 1-5 show the relationship between head and pressure in a centrifugal pump moving liquids with different specific gravities. There is more on this in Chapter 7.

The above graphic shows three identical pumps, each designed to develop 92.4 feet of head. When they pump liquids of different specific gravities, the heads remain the same, but the pressures vary in proportion to the specific gravity.

In the graphic below (Figure 1-5), these three pumps are developing the same discharge pressure. In this case they develop different heads inversely proportional to the specific gravity of the fluids.

The concept of Head versus Pressure causes confusion between maintenance people and the pump manufacturer. The maintenance technician reads his gauges recording pressure in psi, and the pump manufacturer uses the term head. The term head is the constant for the manufacturer. A pump that generates 90 feet of head can elevate water, gasoline, caustic soda, and any liquid to a height of 90 feet. The manufacturer doesn't know the ultimate service of the pump when he manufactures it. He only knows that his pump will develop 90 feet of head. The psi reading is a function of the conversion factor 2.31 and also the specific gravity. This is why you cannot specify a pump by the psi. If the maintenance engineer or mechanic wants to have an intelligent conversation with the pump manufacturer, he must understand and use the concept of 'head'. This is also the reason that too many pumps are sold without adequate gauges. It's somewhat like selling a car without a dashboard. There's more information on this in Chapters 7 and 8.

Given the following information: sp. gr. of water = 1.0

sp. gr. of concentrated sulfuric acid = 2.00 of sea water = 1.03

A pump capable of generating 125 feet of head would provide the following pressures:

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