Keep ejectors pumping

Of all the vacuum-producing devices, the steam jet ejector system is the most forgiving. Knowledge of simple maintenance procedures for locating and avoiding trouble will save time and product in your plant.

PlantServices.com

By Henry Hage

Ejectors are used wherever sub-atmospheric pressures are needed for the manufacture of chemicals, foods, fertilizers, pharmaceuticals or a wide range of other processing applications. They also are used in vacuum refrigeration, metallurgy, filtration, high-altitude simulations and testing. Ejectors are logical components of plants where process steam is available.

Understand types and designs
Steam ejectors are classified as single-stage and multistage. The latter are further divided into condensing and noncondensing types. Multistage condensing ejectors have from two stages to six stages. Intercondensers (surface or direct contact) between stages condense the steam from the preceding stage, which reduces the load to be compressed in the following stage.

A condenser, as part of a steam jet vacuum system, removes condensable vapor ahead of a given ejector stage. The condenser permits use of smaller ejectors, and reduces the amount of steam required. Condenser nomenclature is determined by the corresponding operating conditions and functions.

Four-, five- and six-stage ejectors can achieve suction pressures as low as five microns Hg abs. Under such high vacuum conditions, pressure between the preliminary stages is too low to permit ejector steam to condense, so only the final two stages are fitted with condensers.

Precondensers are used when the absolute pressure of the process is sufficiently high — generally 1.5 in. Hg or higher — to allow condensation with the available water supply. One or more ejector stages remove noncondensables from the precondenser.

Condensers or booster condensers (10.5 in. to 4.0 in. Hg abs) condense process vapor and motive steam from one or more preceding booster ejectors.

The two basic condensers types are direct contact and surface contact. The direct contact (countercurrent, barometric design) condenser:

• Has lower first and installation costs.
• Needs less water to achieve a given vacuum.
• Allows operation at lower absolute pressure because of a smaller terminal difference.
• Requires less floor area.
• Requires little or no maintenance because scaling or solids buildup has little effect on condenser performance.
• Is readily fabricated from corrosion-resistant materials and can be supplied economically with rubber lining.
• Provides safe operation without atmospheric relief valve because of open barometric discharge.

The surface condenser, on the other hand:

• Permits main condenser condensate to be used as cooling water through intercondensers and aftercondensers, so ejector steam can be returned to boiler to conserve energy.
• Allows recovery of process product as condensate or gas at higher pressure.
• Doesn’t contaminate water.
• Reduces vacuum surges, which may carry water back to the process.
• Requires less headroom than the direct contact type.

The simplest vacuum system to maintain and troubleshoot is an ejector. Because ejectors have no moving parts, maintenance consists of replacing worn nozzles or diffusers, changing gaskets and cleaning condenser tubes. Troubleshooting a large, multistage system sometimes entails disassembly to check each stage and interstage condenser separately. However, very often the internal problem is located in the last or “Z” stage, which is the first to be disassembled.

COMMON EXTERNAL PROBLEMS Low or (occasionally) excessively high steam pressure. Wet steam. Change in cooling water temperature. Insufficient cooling water flow. High discharge pressure. Fluctuating water pressure. Excessive air leakage. Change in load.

Go over the installation
The steam-jet ejector is a single-point design, meaning it’s most efficient at only one specific operating condition. If selected and installed properly, and supplied with specified utilities, the ejector system should operate trouble-free for many years. If it’s giving trouble, check the installation to be sure operating variables are correct.

For example, dry steam of the correct pressure is a basic requirement for good performance. Be sure the inlet motive steam header is well trapped. Measure the steam quality. Route the steam line to the ejector from the top of the steam header. Steam lines, traps and separators should be insulated completely to prevent condensation.

Because precondensers and intercondensers operate under vacuum, inlet water pressure isn’t critical, but a barometric condenser needs sufficient pressure differential to control water flow and to overcome pressure drop through surface condensers. Water pressure should remain fairly constant because wide fluctuations change the water flow rate and condenser operator pressure.

TYPICAL INTERNAL TROUBLES Eroded or corroded parts, especially ejector nozzles and diffusers and intercondenser water nozzles Clogged nozzles, diffusers or strainers Clogged or fouled condenser tubes Clogged barometric legs Leaks in steam chest Cracked or worn parts. Clogged or fouled water supply. Clogged water discharge. Excesive lekage due to cracked or worn parts. Eroded intercondenser water nozzle.

Page 1 of 2

<< < 1 | 2 > >>

Sponsored Links