December 2010, Vol. 22, No.12

Operator Essentials

What every operator should know about pressure

Woodie Mark Muirhead

( Click here  for a PDF of this article.)

Knowledge

Principle

A Practical Consideration

Conversions

There are many pressure unit expressions, but two of the most commonly used are pounds per square inch (lb/in.2) and pascal (Pa). Pascal is from the International System of Units (SI), which is the modern metric system.

1 Pa = 0.00014 lb/in.2

1 lb/in.2 = 6964.8 Pa

We use lb/in.2 commonly in the U.S., whereas most of the world uses SI units, including Pa for pressure.

Head

Head is the pressure exerted by a column of water as measured in feet or meters and also the energy in a pumped system developed by a pump.

A 1-ft × 1-ft × 2.31-ft container can hold 2.31 ft3 of water. Given the following conversions,

1 ft3 = 7.48 gal and 1 gal = 8.34 lb,

2.31 ft3 of water weighs 144 lb. Since the bottom of the container is 1 ft2 or 144 in.2, the weight of the water exerts 1 lb per in.2 (lb/in.2 ). Therefore, 2.31 ft of head equals 1 lb/in.2 .

Head loss

Head loss is the energy required to drive a fluid through a particular element or a series of elements (lengths of pipe, fittings, filters, valves, etc.) at a specific rate of flow.

In an effluent filter, head loss is the reduction of head by friction or turbulence as effluent flows through the media. Head loss is used as a measure of effluent filter cleanliness. The media in effluent filters restrict the flow of water, creating head loss. As solids accumulate in the filter, the head loss increases. When the filter is backwashed to remove the solids, the head loss decreases.

Total dynamic head (TDH)

TDH is the energy rating of a pump for a particular rate of flow. The required energy is a function of static head and dynamic losses.

  • Static head is the vertical distance from the water level at the source to the highest point where the water must be delivered.
  • Dynamic losses (or friction losses) are associated with turbulence, length of pipe, pipe material roughness, and downstream appurtenances (e.g., valves and fittings) that cause turbulence in the liquid flow and result in more energy being needed to achieve a specific rate of flow in a system.

TDH is an essential design aspect of all pumping systems. As the name implies, it is dynamic and changes regularly based on a number of variables. If treated effluent is pumped through a long ocean outfall, the total dynamic head can change.

  • The static head will vary depending on the water level at the source of the effluent wet well and on the level of the tides during pumping.
  • Friction losses will vary based on the rate of effluent being pumped and, to a lesser extent, due to biological growth on the interior of the outfall pipe.

Differential pressure

Differential pressure is the pressure difference at two points for the same liquid or gas.

Preliminary treatment mechanical screens are often controlled to start and stop based on the difference in the upstream and downstream channel water level. As debris accumulates on the screen, the upstream water level increases, resulting in a greater differential pressure, and signaling a need to clean the screen.

Differential pressure also is the means by which venturi flowmeters work. Pressure is measured in a pipe and in a constriction in the pipe. The relationship between the differential pressure and the respective areas of the pipe where the pressure is measured is used to calculate flow.

Absolute pressure versus gauge pressure

Gauge pressure is the pressure as measured by a pressure gauge. Absolute pressure is the total pressure exerted on a system, which is gauge pressure plus atmospheric pressure.

If we measure the pressure in a tire, and the gauge reads 60 lb/in.2 , the tire pressure is reported as 60 lb/in.2.  Atmospheric pressure varies with altitude and temperature. If the atmospheric pressure is 14.7 lb/in.2 , the absolute pressure of the same tire is 74.7 lb/in.2 .

Henry’s Law

At a constant temperature, the amount of a particular gas dissolved in a specific type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid.

Dissolved air flotation thickeners (DAFTs) rely on this principle. A pressurized air cushion is maintained above water in a saturation tank. This causes oxygen, carbon dioxide, and nitrogen to saturate the water. When the air-saturated water is released into the DAFT at near-atmospheric pressure, the gases come out of solution and form bubbles that attach to the solids, causing them to float.

The covers of anaerobic digesters and pure oxygen activated sludge tanks trap the carbon dioxide released by biological activity, increasing the partial pressure of the gas above the liquid.  This allows more carbon dioxide to dissolve in the liquid to form carbonic acid and influence the pH.

Boyle’s Law

The pressure and volume are inversely proportional for a fixed amount of an ideal gas at a fixed temperature.

As air bubbles that are released from diffusers rise to the surface in an aeration basin, they increase in size. This happens because there is less water pressure pushing on the sides of the bubbles near the surface than at the bottom of the tank.

Woodie Mark Muirhead is a vice president and operations specialist in the Honolulu office of Brown and Caldwell (Walnut Creek, Calif.).

 

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