How It Works: Pressure Loss

Pressure loss, also known as pressure drop, is the decrease in pressure as measured between two points in a flowing fluid system. Pressure drop occurring along the direction of flow in a pipe is caused by fluid friction, both internal in the fluid itself, as well as with the piping surfaces, piping restrictions or sudden changes in the geometry of the flow path. Pressure loss is directly related to fluid velocity, specific gravity, viscosity and the size, shape, and roughness of the pipe interior.

The flow rate determines the pressure loss. Up to certain limits, the higher the flow rate, the greater the pressure drop, and vice versa. It is important to note that pressure drop across a valve is not created by the valve alone.

Pressure drop is a critical element in valve sizing and valve selection. Potential pressure drop must be known by the engineer designing the system in order to ensure proper valve selection. “The traditional way of determining potential valve pressure loss is to set up what is commonly referred to as a flow loop,” David Atkinson, Cameron product manager for check valves, explains.

During a flow loop test, a pump is used to move 70° F water through the valve and then return it back to the pump to start all over again. This measures the pressure loss downstream of the valve(s), using a gauge. This initial technique established the flow calculations that are still being used today to determine the pressure loss across various valves.

With the introduction of computers, it is now possible to calculate the pressure loss across a valve without a flow loop. A program called Finite Element Analysis (FEA) allows an engineer to set up a simulation by entering the relevant data on the valve interior and then simulating flow through the valve.

“This program will give the same results as a flow loop, but without having to buy all the pipe, fitting and pumps for the various valve sizes to produce the system in real life,” Atkinson said.

Pressure loss is expressed in two ways; through a flow coefficient (CV) value, or an equivalent length of pipe. CV expresses flow rate in gallons per minute (gpm) of water at 70° F, with a 1 psi pressure drop across the valve, when the valve is in the full open position. Equivalent length of pipe converts the pressure drop to the equivalent pressure drop incurred in a length of pipe operating under the same volumetric and pressure conditions.

Cameron offers valves that have low pressure drop, like the CAMERON T31 fully welded ball valve. Because the valve opens and closes at a constant rate, the T31 ball valve has low pressure loss.

What is a pressure relief valve?

Pressure relief valves, also known as PRV, are used to control or limit excessive pressure in a system. A PRV is designed to discharge and reduce pressure in the event that a pressurized system has a rise in pressure beyond preset design limits. The PRV serves as an outlet for the dangerous buildup of pressure. When the pressure in a system becomes too high, the pressurized fluid will vent from the PRV. The valve opens at a set pressure point so, when the pressure in the system exceeds this point, it automatically opens to relieve a portion of the fluid pressure. As the fluid is discharged, the pressure inside the system will stabilize and the valve will close.

What is a pressure regulator?

A pressure regulator is a valve that automatically modulates the flow of fluid in order to maintain a certain pressure. Used in high pressure situations, regulators, in conjunction with other equipment, allow fluid supply lines or tanks to be safe for various applications.

There are three elements to a pressure regulator: a restricting, a loading, and a measuring, element. The restricting element is a valve. It can be any valve that can operate as a variable restriction to the flow, such as a globe or butterfly valve, i.e. a valve that can throttle flow.  The Cameron NEWCO line of forged and cast globe valves are commonly selected for this application, having a proven track record in the field. The loading element is the needed force for the restricting element, i.e. the way to operate the valve. It can be a weight, spring, piston actuator, etc. Finally, the measuring element determines when the pressure of the flow coming into the line requires modulation of the restricting element (cycling of the valve) to properly control the desired pressure.