Hand Expansion Valves – 1

We have been somewhat busy the last few weeks completing a new project.  The next few blog entries should be more timely.

These valves are used for multiple purposes in ammonia refrigeration systems.  However, they are commonly designated by a single description.  This common term of hand expansion valve (HEV) is sometimes misleading. It may help to consider the actual function the valve provides; throttling of liquid or vapor and the mass flow requirements for the application.

The common applications of this valve are:

• Liquid make-up to maintain a relatively constant liquid level in a vessel.
• Throttling liquid from a higher pressure to a lower pressure (control pressure systems).
• Balancing upstream liquid pressures to evaporators in a liquid overfeed system.
• Throttling vapor flow in float columns to minimize rapid liquid level fluctuations.

The next few blog entries will discuss the multiple applications of these valves.

Hand expansion valves used for liquid level control operation
In this service the hand expansion valve throttles the liquid supply pressure down to the pressure within the vessel.  Whenever the liquid solenoid valve is open to add liquid to the vessel the hand expansion valve capacity/setting is dependent upon the pressure differential across the valve needle and seat.

In addition, the valve capacity required is also dependent on the duration that the liquid solenoid is energized (open).  As an example, if the liquid make-up capacity required is 50 tons to maintain the liquid level you also have to consider the time duration the valve is open.  Therefore if the hand expansion valve is adjusted to provide a duration of 3 minutes of energized operation (out of a 5 minute cycle for instance) for the liquid line solenoid the valve capacity required becomes 50 X 5/3 = 83.3 Tons.

This corrects the valve capacity and line sizing to allow for the actual time the liquid line solenoid is energized.  Don’t make the assumption that the liquid line solenoid and hand expansion valve capacity are based on the actual requirement (50 Tons in this example) as this would be considered as the liquid line solenoid is open 100% of the time, unless a modulating valve is used to maintain the liquid level.

Secondly, it is important to recognize the valve size selection should be based on the valve capacity when the valve is approximately 50% open.  This allows some flexibility to adjust the valve during actual operation.  After all, if the valve is selected for capacity at 100% (valve fully open) you do not have any further adjustment to account for variations in operation.

Tags: controlled pressure systems, float columns, hand expansion valves, liquid overfeed, operating issues

This entry was posted on Sunday, September 27th, 2009 at 8:06 pm and is filed under ammonia refrigeration components, industrial refrigeration. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.