Velocity through check valve - max and min 1

[alexcmmi]

A supplier brought to my attention that I had an unusually high velocity going through a check valve in a PS design I was working on...

Pump is split case, 10" suction x 8" discharge. Design point is 2604 gpm at 177'. Service is raw (high quality) water, 60 deg F - nothing out of the ordinary.

Coming off the pump flange face, I have a 90 bend, and then check after the bend. I increase to my line size after the check.

My line velocities are fine, however, the velocities through the check are rather fast (15 fps at operating point, - higher at peak flows).

What is typical max velocity through a check?

Further, while the pumps are VFD driven, the nature of the application is that the flows could be low - with cycling at off peak times. What is the MIN velocity needed so that I dont get 'flutter' (not sure of the word) of the check?

Probably use a tilted disc type or air cushioned swing.

Thanks in advance

 

[gerhardl]

A check valve should correctly be dimensioned by flow (eg fluid velocity) rather than dimension, both to give the smallest flow loss and to have the best soft closing properties to avoid pressure peaks when closing.

By variable flow (soft stop and start by pumps, and motor regulation of flow) we see increasing cases where check valves are 'mis-selected' and causes problems and unnecessary large power losses when pumping.

You will need a certain straight length before and after the check valve at this velocity to have a proper laminar even flow.

If you select a valve 'from the upper shelf', for example a soft closing nozzele check valve similar to or as sample given below, you will find that 3 to 4 m/s ( 9 to 12 feet a second - see brochure for max) is a normal flow , and that the very low pressure loss may give a very reasonable payback time for the extra cost of the long-life construction.

Water mains are normally lower than this, about 1 -2 m/s, 3 m/s when cleaning.

Hydre-electrical power plants will however have higher velocities, also when pumping water back to store against times when energy is higher priced, but even then a nozzle check valve should be limitied to max 5- 6 m/s,

You will also find that the spring operated construction will ensure that the the valve is closed at the instant when the pressure at both sides are equal, and pressure peaks will be minimized, tolerence for varying flow maximized.

http://www.erhard.de/index_e.html

go to downloads, online leflets and non-slam nozzle check valve brochure

 

[Johnnybv]

The typical min/max velocity through a check varies on the type of check. Ultimately you want the check to get to a fully open position as soon as possible and to close before flow reversal.

I like to take the position that a check valve is like a flow activated control valve. A perfect check valve is fully open at the instant of positive delta p and as soon as flow velocity is 0 it closes instantly. All check valves try to attain this impossible standard. Some valve manufacturers will post their min flow velocities for fully open status. Keep in mind that posted Cv's on check valves are almost always for fully open check valves. Ultimately, the flow in your system may not be enough to fully open some check valves - you'll have to look around. Personally, for a clean water service I like wafer style swing checks (opposed to dual doors) with external options (springs, hydraulics, limit switches, etc.) Provided you have the minimum velocity to fully open the wafer, which with the right options can be almost 0, the result is a fast opening stable disc in the fully open position.