Graham Corporation Technical Article: Ejector Motive Steam Pressure

Ejector Motive Steam Pressure

November 2021

Greetings!

This article is going to discuss ejector motive steam pressure. The correct motive steam pressure is important to allow the ejector to operate properly. When designing an ejector, the motive steam pressure provided when sizing the ejector needs to be the lowest pressure the ejector will see. Also if there is a long piping run from the steam header to the location of the ejector the pressure drop needs to be considered before designing the ejector.

Refer to our previous article on pressure drops, click on the link to go to the article:

Piping Pressure Drop and Ejector Vacuum System Performance Article

The ejector motive steam pressure should be no less than design and no more than twenty percent above design.

BEST OPERATING MOTIVE STEAM PRESSURE

-0% Below Design and +20% Above Design

The ejector motive steam provides the energy for compression. The ejector is designed with a certain mass flow rate of motive steam at a certain pressure and temperature. If the ejector does not have the right conditions, it will not operate properly. The quality of the motive steam is also important and should never be wet.

Refer to our previous article on wet steam, click on the link to go to the article:

Wet Steam and Ejectors Article

The motive steam pressure below design does not provide enough energy for the ejectors to operate and will cause the shockwave to break. If the ejector has higher than design motive steam pressure, then the capacity is reduced and steam is wasted.

Refer to our previous article on motive steam consumption, click on the link to go to the article:

Ejector Motive Steam Consumption Article

Below is a simplified example of what happens with higher than design motive steam pressure. Higher than design pressure reduces the capacity and in the example below we use units for easy reference. The design uses 50 units of motive steam pressure at 100 psig. Increasing the motive steam pressure to 150 psig changes the entering steam to 70 units. Since the ejector is designed to handle a certain amount with fixed geometry, an increase in motive steam pressure increases the motive steam entering the ejector and in turn reduces the suction load. In the example the suction load goes from the design 50 units to 30 units due to the increase motive steam pressure.