Page 189 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Compressor Best Practices Best Practice 3 .14
- High density propylene There is no question about it, solvent injection is expensive!
- Fluid catalytic cracker off gas (wet gas) Most solvents used are naphtha based. Many users have required
- Thermal catalytic cracker off gas (wet gas) flow meters at each injection point to determine the proper
- Coker gas amount of injection. Typically, the total amount of solvent in-
jection in one compressor case should be 1e2% of mass flow,
The temperature below which fouling can be prevented but the total amount should never exceed 3% of weight flow.
varies with each process, compressor and application. Moni-
Excessive solvent injection will erode leading edge blade tips at
toring of process conditions is necessary to establish a threshold the blade to cover junction on closed impellers and can cause
temperature in each case. In some cases, fouling cannot be impeller failure if not detected in time.
prevented with the existing compressor. It may be necessary to 5. Impeller or blade coatings e In the last few years, some
modify the aerodynamic design and/or add additional cooling. users have investigated and have used impeller and blade row
Note that water injection is used quite regularly in ethylene unit coatings to reduce friction in lieu of continuous solvent in-
cracked gas compressors to reduce gas temperature. However, jection. Based on my experience, this has met with mixed re-
an industry best practice is to keep the amount of injection sults. There has been success with coating impellers with
water below 3% of the inlet mass flow. Teflonebased material in ethylene cracked gas service. It is
2. Proper inlet filter selection and maintenance e In air recommended that the process designer and compressor vendor
compressor and gas turbine applications, filter selection is an be contacted for additional information in this regard.
important factor in preventing fouling. Site conditions must be
accurately monitored prior to installation. It is advisable to
contact filter vendors to survey site conditions. Filters must be Corrective measures
able to prevent debris and excessive moisture from entering the
air compressor. In platform applications and installation close to If fouling cannot be immediately prevented, various off-line and
water, special designs must be used to prevent excessive chlo- on-line cleaning procedures are available. Off-line cleaning
ride (salt) build-up. It is likely that multi-stage air filtration methods are definitely preferred to on-line cleaning methods,
should be considered. Fouled air compressor sections of gas since they do not expose the compressor to vibration excursions
turbines can reduce output power by 3e5%. and have proven to be more effective if a compressor is severely
Another important consideration is filter maintainability. For fouled. Failure to immediately remove all foulant from the rotor
crucial (un-spared) services, filters with on-line cleaning capa- can cause catastrophic failure, since a large amount of unbalance
bility should be considered. In recent years pulse type (“Huff can be instantaneously introduced into the rotor system. Gen-
and Puff”) filters that use a reverse air pulse to clean filter car- eral guidelines for an off-line or “crank” washing procedure are
tridges when filter pressure drop becomes excessive have presented in Figure 3.14.12.
become popular.
Regardless of the type of air filtration used, or not used, re-
member environmental conditions can change as previously 1. Determine proper solvent by solubility test of foulant material.
discussed in this module. 2. Confirm solvent does not harm compressor internals (aluminum
3. Condition monitoring of compressor parameters e Re- or Teflon-based labyrinth).
gardless of the service, condition monitoring of molecular 3. Fill and vent case to be sure it is completely filled with solvent.
weight, pressure and temperature will provide valuable in- Note: use warm solvent, not to exceed 120°C (250°F) if
formation regarding process condition changes that can cause or possible. Be sure to confirm solvent is acceptable for use at this
temperature.
increase fouling. 4. Use turning gear or small belt drive assembly to turn case at 40–
4. On-line solvent injection e Certain process applications 60 rpm during wash.
have had success with on-line solvent injection. The objective of 5. Measure conductivity of solvent solution prior to start of wash.
this measure is to continuously inject a small amount of solvent 6. Change wash fluid frequently and continue wash until
to reduce the friction coefficient of the blade and impeller conductivity of wash liquid is equal to conductivity of initial
surface and thus prevent the fouling from accumulating on the value measured in step 5 above.
surface. The object of solvent injection is often misunderstood.
The idea is to prevent foulant accumulation, not clean the im- Fig 3.14.12 Off-line cleaning guidelines
peller or blade on-line. Non-continuous solvent injection will
allow the impeller or blade surface to dry and thus promote If an on-line wash is to be used, it should be employed peri-
fouling. Even if a unit injection is continuous, selection and lo- odically e usually at least once a week e and started when the
cation of injection spray nozzles is a critical factor in success. rotor is in new (clean) condition. As stated previously, the
Insertion of spray nozzle(s) in the inlet pipe of a compressor injected amount of wash fluid should not exceed 3% of mass flow.
section is usually not as effective as injection directly into each Process and compressor designers should be consulted regarding
stage of the compressor. Knowledge of solvent vapor pressure the exact procedure to be used. It is advisable that a radial vi-
and internal compressor temperatures is necessary to determine bration trip system be employed if on-line washing procedure is
if section or stage solvent injection is required. to be implemented. This action will prevent catastrophic com-
Type of spray nozzle to be used can best be determined by pressor failure in the event of an incomplete on-line wash.
discussions with process designers or other users having similar We have included some examples of fouling along with
process applications. preventive measures and cleaning methods in Table 3.14.1.

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