How to Calculate Pressure Drop Across a Valve?


There are many valves installed in a chilled water system. Every valve has its own pressure drop which the chilled water pump must overcome to supply enough flow to all air handling units. So, how do you calculate pressure drop across a valve?

In short, you can use the formula ∆p = 0.5Kρv2 to calculate pressure drop across a valve. The K in the formula is the valve K factor which you obtain by referring ASHRAE handbook of fundamental.

Calculating valve pressure drop is part of the total pressure drop calculation. If you haven't got the pressure drop of the chilled water pipe, I encourage you to check out my post on how to size chilled water pipe.

In case you are not sure what is meant by the pressure drop across a valve, it is basically the difference in water pressure between before going into a valve and after coming out of a valve.

“The pressure difference between the inlet and outlet of a valve is called pressure drop.”

Pressure drop is caused by the obstruction and the friction of the valve. Valves have different shapes and designs which can cause different amount of resistance and thus, different amount of pressure drop.

Calculating Valve Pressure Drop

There are a few formulas that can be used to calculate pressure drop but I find it is easier to use calculate pressure drop across a valve by using the following formula:

∆p = 0.5Kρv2

where,
∆p = Pressure drop, Pa
K = Valve K factor
ρ = Density of water, 997 kg/m3
v = Velocity of water, m/s

Pressure drop across a valve equals 0.5 times K factor times density of water times velocity of water squared. The result is in Pascal (Pa) but we usually divide it by 1000 to get a more practical figure in kilo Pascal (kPa).

PressureConversion
1 kPa1000 Pa
100 kPa1 bar
100 kPa14.5 psi
10 kPa1.02 m of head
Pressure Conversions Table

Usually, pressure drop results in meter of head are more useful as adding all the meter of heads of valves, fittings, pipes and other pieces of HVAC equipment will let us know the required pump head.

Organizations such as ASHRAE and Carrier had published their recommended velocity for chilled water systems. The higher the water velocity, the greater the erosion on pipes.

Below is the recommended water velocity:

Normal Operation (hr/yr)Water Velocity (m/s)Water Velocity (ft/s)
80002.48
60003.010
40003.712
30004.013
20004.414
15004.615
Recommended Water Velocity for Chilled Water Systems

I generally use 10 ft/s or 3 m/s of water velocity when designing the size of pipes for chilled water systems. Since the pressure drop formula includes the velocity of water squared, pressure drop increases exponentially with water velocity.

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So, the higher the water velocity, the greater the pressure drop.

Different types of valves have different pressure drops. Valves with thread connections usually have a lower pressure drop than valves with flange connections.

However, valve sizes above 50mm are mostly using flange connections because it is often cheaper and more practical to install.

If you want to learn how to calculate pump head, see my post How to Calculate Chilled Water Pump Head?.

Which Valve Has More Pressure Drop?

Globe valves generally have more pressure drop than gate valves. A 50mm threaded globe valves have 41 times more pressure drop than 50mm threaded gate valves at 3 m/s of water velocity.

On the other hand, gate valves generally have very little pressure drop.

Personally, I take globe valves as big-sized gate valves just like butterfly valves are big-sized ball valves. It helps me to remember better because when you need isolation valves on a pipe size bigger than 50mm, you use butterfly valves instead of ball valves.

When compared to gate valves, angle valves and swing check valves, globe valves have the highest pressure drop. However, globe valves are very good for flow control. That's why pressure-independent control valves are mostly globe valves.

In the past, I had experiences with two valves connecting side-by-side (eg: a ball valve to a y-strainer). I noticed that the pressure drop increases significantly, out of the chart. Hence, something to be aware of.

Usually, we put a 150mm short pipe in between valves and fittings to avoid turbulent flow that may increase the pressure drop significantly. In addition, valves such as butterfly valves have an orifice that required some space clearance to open.


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