To achieve the most efficient operation of a steam system, controlling the steam flow is essential.  This requires varying the rate of steam entering the system.  In most vacuum systems this control is achieved by the main steam valve.  This could be a weight and lever valve, a motorized steam valve or in some cases just a globe valve (a gate valve should not be used for throttling service).

To understand the need for a control valve we must go back to the principles of a vacuum system.

Most steam heating system operators believe that the vacuum pump creates the vacuum in the system.  This is a misconception.  The condensing in the radiators produces most of the vacuum.  Additionally, the vacuum pump creates a differential pressure at the end of the condensate piping, aiding the flow of condensate out of the radiators back to the condensate receiver.

The vacuum occurs when the condensing rate of the system is greater than the steam flow in to the system.

The steam leaving the boiler in the average steam system is in the 5 to 7 psi range.  This corresponds to a steam temperature of 226 to 234 degrees Fahrenheit flowing into the radiators.  If the volume is uncontrolled, the steam flowing into the radiators and the condensing rate stays close to or remains the same as the steam temperature of 226 to 234 degrees Fahrenheit.  Most of the time this is much too hot and as a result it causes overheating.  Most likely the user will usually end up shutting off the radiator hand valve, leaving the space without any heat at all.

By reducing the steam flow into the system at less than the condensing rate, a vacuum is created.  Using the steam temperature table a vacuum of 20″mg will result in a temperature of 161.19 Fahrenheit.  A tight system with no leaks should be able to produce this kind of vacuum.

Under these conditions the most efficient operation is achieved.