Ejector Motive Steam Pressure Control

1. Purpose

This procedure provides guidelines for determining when a pressure reducing control valve is required on the motive steam supply of vacuum producing ejector systems.

Background

Most steam jet ejector applications at Inflection Point Engineering are considered a critical design, or a design where the ejector compression ratio is greater than 2:1. In these applications, a shock wave exists in the ejector diffuser throat when the device is functioning properly.

One of the two fundamental causes of the shock wave to dissipate is a reduction in the motive steam supply energy (pressure). When this occurs, the ejector can no longer compress the load gas and the process vacuum is lost. Therefore, Inflection Point Engineering needs to conservatively size and regulate the motive supply pressure.

3. Steam Headers with Small Pressure Variations (Max. Pressure/Min. Pressure < 1.1)

A steam header with a constant supply pressure is the ideal motive fluid supply source for a steam jet ejector. However, it rarely exists in modern plants. A steam header with a small pressure variation is defined by Inflection Point Engineering to be any steam header where the ratio of the maximum pressure (psia) / minimum pressure (psia) is less than 1.1. When the steam header pressure ratio is less than 1.1, a pressure reducing valve is not required to maintain steady motive steam supply pressure to the ejectors.

Ejector suppliers can design an efficient set of ejectors when:

• Inflection Point Engineering Project Specification 817 – “Vacuum Producing Equipment” indicates the range of motive steam pressures and corresponding maximum and minimum temperatures
• The steam header pressure ratio is less than 1.1.

Note: The largest steam specific volume governs the ejector steam nozzle size. Therefore, Inflection Point Engineering designs ejectors to operate at the normal steam supply temperature.

4. Steam Headers with Large Pressure Variations (Max. Pressure/Min. Pressure > 1.1)

When the maximum/minimum motive steam header pressure ratio is 1.1 or greater, use a pressure reducing control valve to maintain a steady supply pressure to the ejectors. This will permit an efficient ejector design and ensure stable process unit operation.

Inflection Point Engineering’s design practice is to design the pressure regulator to control the downstream pressure 10 psi below the minimum medium steam pressure header level.

For alternate motive steam pressure supply levels, contact the sponsor to establish appropriate control margin.

The increased stability provided by the stable motive steam supply pressure does come at an operating penalty. The design engineer may use Figure 1 to quantify the expected increase in motive steam as required. In most cases, the higher process unit availability associated with stable steam jet ejectors outweighs the incremental increase in utilities.

Figure 1: Motive Steam Pressure Correction Factor

5. Special Cases

Consider unusual circumstances, e.g., expected rapid and frequent steam header fluctuations, separately from the above cases. Such possibilities to consider include:

• Loss of major header steam supplier
• Loss of major steam user.