Many times there is piping between ejector vacuum system and process vessel that will generate some amount of pressure drop. This pressure drop needs to be included in the vacuum system’s design so that the actual pressure in the process vessel is correct, see the below example.

Process Vessel Required Pressure 12.8 mmHgA

Piping losses: 2.8 mm Hg

Design Vacuum System Suction Pressure: 10 mmHgA

If the process vessel is required to operate at 10 mmHgA and the piping losses were not accounted for in the initial design, then the vacuum will not be as expected. In this scenario, the process may suffer from poorer vacuum, or the loading to the vacuum system could be reduced to allow the ejector system to pull a deeper vacuum compensating for the piping losses. In either scenario, the system either has less capacity or operates at a poorer vacuum.

More often than not, the ejectors and condensers in a system are very near to each other with short piping runs. The pressure drop through the interconnecting piping is often negligible. However, there are times with the piping runs are longer and the pressure drop is not negligible. It’s recommended that the piping runs all be evaluated and if the piping runs have notable pressure drops, those pressure drops be incorporated into the ejector design. This is done by designing the ejectors to either have deeper suction pressures and/or discharge to higher pressures to compensate for the piping losses, see the example below when pressure drop is not incorporated.

First Stage Ejector Design Discharge Pressure: 75 mmHgA

First Intercondenser Design Inlet Pressure: 75 mmHgA

Pressure Loss in Piping from First Stage

Ejector to First Intercondenser: 2.1 mmHg

Actual First Stage Ejector Discharge Pressure: 77.1 mmHgA

If the piping pressure losses are not accounted for, the discharge pressure of the upstream ejector will be higher by that amount. As the discharge pressure of the ejector is increased, it can cause instability or a severe reduction in performance. The system may be able to compensate for this with additional cooling water flow, a colder cooling water temperature, a higher motive steam pressure, or a reduction in loading. If none of these variables can be improved on, then the first stage ejector would need to be redesigned for the higher back pressure. While interstage piping losses are normally minimal, if an installation has long piping runs or isolation valves between pieces of equipment, those losses need to be accounted for.

Pressure loss in steam piping and variable steam pressure is a very common issue with vacuum systems and vacuum system design. If an ejector system is not supplied with the correct motive steam pressure, the ejectors will not have the energy they need to work correctly. This can cause vacuum instability issues and ultimately a poorer process vessel operating pressure. Just because a steam header is designed for a certain pressure, doesn’t mean that the entire header is going to be at that pressure. Ejector systems are often located some distance from the boiler. While the header pressure at the boiler may be at the header’s design, once piping losses have been accounted for the pressure at the ejector system is almost always lower than this, see the below example.

Ejector Motive Steam Design Pressure: 150 psig

Design Header Pressure: 150 psig

Pressure at Ejectors: 138 psig

Ejector Motive Steam flow at 138 psig: 7% loss of motive flow

If the ejectors are designed for motive at 150 psig but the actual pressure is 138 psig, then the ejectors will see a reduction in motive steam flow of roughly 7%. This is more than enough to cause performance issues. In order to compensate for the lower motive steam pressure, the ejector nozzles would either need to be re-machined or replaced with nozzles designed for the 138 psig motive pressure. This requires that the equipment be shut down for a period of time. Depending on the ejector location, scaffolding or cranes may be needed. Pressure loss in steam piping needs to be accounted for in the initial design. If the motive steam supply will fluctuate, the ejectors should be designed for the minimum pressure that will be provided at the ejectors.

An often self-induced problem in vacuum systems is undersized piping. Undersized piping will increase pressure drop and can cause a vacuum system to not work correctly, which is why it’s important that all process lines and steam lines are sized correctly for their corresponding flows. At deeper vacuum, the specific volume of a fixed amount of vapor increases. To compensate for these changes, the line size of a vacuum system’s piping almost always gets larger as you move to deeper vacuum. This can result in large line sizes for systems that operate at deep levels of vacuum. While larger lines are more expensive and heavier to support, they should not be reduced down to a smaller line size. Undersized piping or restrictions in piping can cause performance issues as outlined in the below example.

If an ejector has a 24-inch suction connection, the suction piping should not be piped down to an 18-inch line. The undersized piping will create additional pressure drop and result in higher velocities. This can ultimately result in poor operating pressures and erosion. To fixed an undersized line, the undersized piping needs to be removed and replaced. The ejector could also be re-designed using more motive steam, which could increase the operating utility cost. During the design process undersized pipes and reducers or restrictions should be avoided. There may also be specific instances where it might be desirable to oversize a process line to minimize the impact of pressure loss.