Our topic for this article is wet steam or poor steam quality and its impact on ejectors and their operation. Ejectors utilize motive steam to generate vacuum. Understanding how steam quality impacts ejectors is important for both the performance and mechanical reliability. This discussion is going to cover both of these topics.

First, let’s define wet steam in a closed system. When liquid water reaches its boiling point, additional heat energy will cause a portion of that water to start boiling off. The liquid water will continue to boil off as additional heat energy is added until all of the water has become steam. When this happens, the steam has reached what is known as the dry and saturated point. Additional heat energy added to the steam beyond that causes the steam to become superheated. On the other hand, if steam at the saturation temperature is allowed to cool, a portion of the steam will condense back into liquid water. Such steam is known as “wet steam”. Steam quality is a measurement of the amount of steam vapor in a saturated liquid / vapor mixture. Steam quality of 100 indicates 100% vapor. Wet steam will have a quality below 100, dry or superheated steam will have a quality of 100. It’s important to note that steam that is dry or superheated at the boiler may not necessarily be dry by the time it reaches the ejectors. This difference is often overlooked in the field.

Wet motive steam has a negative impact on ejector performance. Ejectors have what is called a motive nozzle and that nozzle expands the high pressure motive steam, which creates high velocity steam. The kinetic energy in that high velocity steam is what drives the ejector performance. Steam vapor is compressible, however liquid water is not. As liquid droplets in the wet steam pass through the ejector nozzle, they displace some of that motive steam. Since those liquid droplets can’t be expanded, it ultimately reduces the amount of energy passing through the nozzle which can negatively impact an ejector’s suction pressure and the overall system performance.

Wet steam leaving a motive nozzle at high velocity also can cause reliability issues. The motive nozzle in an ejector generates a high velocity at its discharge that can be several times the speed of sound. When wet steam is passed through the ejector’s nozzle, the droplets of water in the steam are accelerated to very high speeds. These liquid droplets will make contact with the ejector’s diffuser, leading to impingement damage. Over time that impingement will erode the internals of the ejector and the downstream piping elbows or equipment. If left unchecked, the steam will eventually wear a hole through the side of the diffuser allowing air to be pulled into the vacuum system, causing poor performance. On smaller ejectors with cast bodies, the wall thickness is a larger percentage of the cross sectional area of the ejector and you often have a substantial reduction in performance as the ejector’s internal geometry is changed.

How does one identify wet steam and mitigate its effects? Identifying wet steam is very hard to do in an ejector system. Identifying wet steam inside of a pipe using a temperature gun is not possible, since a saturated water steam mixture will be the same temperature. Bleeding low point drains to look for a constant discharge of water can identify some of the more extreme examples of wet steam. One other way of identifying wet steam is by measuring the discharge temperature of the ejector and downstream piping. As water in the motive steam enters the ejector, which is under vacuum, some of that water will flash off. This cools the outlet of the diffuser relative to the normal operating temperature. If the normal discharge temperature for a specific ejector is not known, please consult the ejector manufacturer. One can also identify wet steam based on the condition of the ejectors and maintenance records.

To correct a wet steam issue it’s best to first confirm the boiler is producing good 100% quality steam. If a moisture problem still exists, then it’s developing in the piping between the boiler and the ejector system. This could be due to low steam velocities inside of the steam lines or poor insulation of those steam lines. Verify that all of the steam piping is well insulated and the low points of the steam lines are properly trapped for condensate removal. Pipe branches leading to the ejectors should come from the top of any header lines to prevent liquid from carrying into the ejectors. A cyclonic steam separator installed close to each ejector will also improve the steam quality before entering the ejector.