how does it work_ pressure loss

7/29/2015 Howdoesitwork:PressureLoss

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How does it work: Pressure Loss

Pressure loss, also known as pressure drop, is the decrease in pressure as measuredbetween two points in a flowing fluid system. Pressure drop occurring along the directionof flow in a pipe is caused by fluid friction, both internal in the fluid itself, as well as with thepiping 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, thegreater the pressure drop, and vice versa. It is important to note that pressure drop across avalve is not created by the valve alone.

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

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

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

This program will give the same results as a flow loop, but without having to buy all thepipe, 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 anequivalent length of pipe. CV expresses flow rate in gallons per minute (gpm) of water at70 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 dropincurred in a length of pipe operating under the same volumetric and pressure conditions.

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7/29/2015 Howdoesitwork:PressureLoss

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Challenges

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

What is a pressure relief valve?

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

What is a pressure regulator?

A pressure regulator is a valve that automatically modulates the flow of fluid in order tomaintain a certain pressure. Used in high pressure situations, regulators, in conjunction withother 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 variablerestriction to the flow, such as a globe or butterfly valve, i.e. a valve that can throttle flow. Camerons NEWCO line of forged and cast globe valves are commonly selected for thisapplication, having a proven track record in the field. The loading element is the neededforce 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 pressureof the flow coming into the line requires modulation of the restricting element (cycling ofthe valve) to properly control the desired pressure.

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7/29/2015 Howdoesitwork:PressureLoss

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