What is control valve sizing?

The process of “control valve sizing” is a procedure where the dynamics of the system are matched to the performance characteristics of the valve. This produces a control valve of an appropriate size and type that will best meet the needs of managing flow within the process system.

Why is a control valve sizing important?

Why is control valve sizing so important? An undersized control valve doesn't have the capacity to pass the required flow. But, more often than not, a control valve is too large for the application. An oversized valve is very sensitive to operating conditions.

What is Cv in control valve sizing?

Valve Flow Coefficient (Cv) is the flow capability of a control valve at fully open conditions relative to the pressure drop across the valve. It is defined as the volume of water (GPM in the US) at 60°F that will flow through a fully open valve with a pressure differential of 1 psi across the valve.

What is over sizing of control valve?

If your control valve seems to be unstable or erratic and the valve is opening and closing constantly, a likely cause is that the valve is oversized. This means the valve is trying to find your set point, but because it is too large it is unable to precisely meet your desired flow or pressure set point.

How do I calculate valve size?

The volume flow rate for liquids can be calculated by multiplying the fluid velocity times the flow area. Thus, Cv is numerically equal to the number of U.S. gallons of water at 60°F that will flow through the valve in one minute when the pressure differential across the valve is one pound per square inch.

Control Valve Sizing Basics: What is Pressure Drop?

What is xT in control valve sizing?

For compressible fluids, a terminal pressure drop ratio, xT, similarly describes the choked pressure drop for a specific valve. When sizing a control valve, the smaller of the actual pressure drop or the choked pressure drop is always used to determine the correct Cv.

How do I choose a control valve?

When choosing a control valve for a process, there are many things that must be considered, including the valve's flow characteristic, its size, noise, the potential for cavitation or flashing damage, body and trim materials, actuator size and type, and its dynamic response to changes in the control signal.

What is cavitation in control valve?

Cavitation happens in control valves when there's variation in the flow velocity, in particular, when the fluid in the valve accelerates rapidly. It occurs when the pressure drop in the valve exceeds a critical point or when downstream pressure exceeds the fluid vapor pressure.

Why is a control valve one smaller?

The idea is to use as much of the valve's control range as possible while maintaining a reasonable, but not excessive, safety factor. If the valve is too small, it will be obvious immediately, as it will not be able to pass the required flow. In actual practice, under sized valves are fairly uncommon.

What is port size in control valve?

Port size is the size of the opening that fluid can travel through when the ball valve is in the open position.

What is Cv calculation?

The formula for the coefficient of variation is: Coefficient of Variation = (Standard Deviation / Mean) * 100. In symbols: CV = (SD/x̄) * 100. Multiplying the coefficient by 100 is an optional step to get a percentage, as opposed to a decimal.

What is Cv and KV in control valve?

The Valve Coefficient (Cv – in Imperial unit) – the number of US GALLON PER MINUTE of water at 60 °F that will flow through a valve at specific opening with a pressure drop of 1 psi across the valve. The flow factor (Kv – in Metric unit) – the amount of water that will flow in m3/hr.

What is Cv value?

What is Cv value? The valve flow coefficient according to the JIS standard, represents the flow capacity in US gallons/minute of 60° F pure water when it is flowing through the valve with a pressure difference of 1 psi at the specified travel (operation range).

How do you calculate Cv flow?

The equation for calculating the valve flow coefficient is Cv = Q √ (SQ/P).

What is valve Cv rating?

The Cv rating for each valve is listed in the tables found in the valve catalog. The definition of Cv is the # of gallons of water that will flow through the valve with a 1 PSI pressure differential when the valve is open. The equations below can be used to determine: Flow Rate, given the Cv and ΔP.

Does the size of the valve matter?

There's no harm in using a valve longer than you need, although it might look a bit odd to use a particularly long valve on shallow wheels. If your wheels are even deeper than the longest valve you can find, you can buy valve extenders to add length to your valve.

How is control valve authority calculated?

Mathematically it is easy to see how, as the authority of a control valve is defined as: N = DPv / (DPv + DPc). Where: DPv = Pressure drop across a fully open control valve DPc = Pressure drop across the remainder of the circuit.

What is Sigma in control valve?

Sigma is the most widely-accepted and precise cavitation index used to quantify and predict cavitation in control valves. Simply put, Sigma is the ratio of the potential for resisting formation of vapor bubbles to the potential for causing formation of vapor bubbles.

What is choked flow in control valve?

Choked flow is the point at which decreasing downstream pressure will not increase the flow through a valve. This typically happens in high differential applications in a high pressure control valve in gas back pressure or pressure reducing service.

What is the difference between flashing and cavitation?

Flashing is an in- to- out event while cavitation is an out- to- in event. Flashing occurs at low pressure and cavitation occurs when the low-pressure bubbles collapse at the high-pressure region within the pump casing producing a shock wave. Reduction of volume of the vapor sphere concentrates matter and energy.

What is Cv in gas flow?

Valve Flow Coefficient (Cv) is a valve's capacity for a liquid or gas to flow through it. It is technically defined as “the volume of water at 60°F (in US gallons) that will flow through a valve per minute with a pressure drop of 1 psi across the valve.”

What is critical flow factor?

Critical Flow Factor, often referred to as “Cf” is a coefficient that defines how pressure will recover after it drops to its lowest point inside the control valve. This lowest point is also referred to as the vena contracta*.

Is higher or lower Cv better?

The lower the value of the coefficient of variation, the more precise the estimate.

Does higher Cv mean higher flow?

The valve coefficient, Cv, is a number which represents the capability of a valve (or any flow component) to flow a fluid. The larger the Cv, the larger the flow at a given pressure differential.