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Wet Stacking and Load Bank Testing

Most generator companies understand the negative impact of prolonged no-load generator operation and are aware of WET STACKING. This condition occurs when diesel engines operate at light loads and low temperatures for prolonged periods. Having worked in the generator industry for over 35 years, I have seen many cases of Wet Stacking, some severe. The photos I have shared are of an Emergency Standby Gen-Set here in Austin, Texas which ran for several weeks due to a main electrical buss failure. The customer had no choice but to run the system for an extended period of time under very light loads. As was the case for this customer, the solution of an extended load bank test usually restores an engine to its designed level of performance. However, leaving a system un-tested or under-tested until such visible conditions exist are risky. In fact, once a Building Engineer observes the oily sludge running down the side of his engine, it’s probably too late…the generator system performance is already compromised!

pic2Yes, the impact on engine performance can go undetected for years when generator testing is consistently performed without connecting building loads or without the use of supplemental loads such as load banks. Because there have been cases of engine wet stacking that have resulted in generator failures, especially in Life Safety applications, specific codes have been developed and implemented by folks like the National Fire Protection Association (NFPA), whose code for Emergency and Standby Power Systems (NFPA110) contains guidelines for proper generator testing. Our Industry Association, the EGSA or Electrical Generator Systems Association works with the NFPA and others on codes and standards to develop best practice. EGSA also provides an important reference book, The EGSA On-Site Power Generation Reference Book.

In the NFPA 110 (2013 Edition) we read, in part:

  • 8.4 Operational Inspection and Testing
  • 8.4.2* Diesel generator sets in service shall be exercised at least once monthly, for a minimum of 30 minutes, using one of the following methods: (1) Loading that maintains the minimum exhaust gas temperatures as recommended by the manufacturer, (2) Under operating temperature conditions and at not less than 30 percent of the EPS standby nameplate kW rating
  •* Diesel-powered EPS installations that do not meet the requirements of 8.4.2 shall be exercised monthly with the available EPSS load and shall be exercised annually with supplemental loads at not less than 50 percent of the EPS nameplate kW rating for 30 continuous minutes and at not less than 75 percent of the EPS nameplate kW rating for 1 continuous hour for a total test duration of not less than 1.5 continuous hours.

And in the EGSA On-Site Power Generation Reference Book (4th Edition) we read this:

Wet stacking occurs when “unburned fuel (is allowed) to accumulate in the exhaust system, which can foul the fuel injectors, engine valves, and exhaust system and reduce the operating performance” and “…the engine generator must not be thought of as a single piece of equipment but as an interconnected system of components…By utilizing a properly designed load bank….the functional capabilities of the power system can be determined”. (page 414)

img-140212022756In fact, in addition to preventing wet stacking, load bank testing also proves each component of a backup generator system including cooling systems, governor systems, voltage regulators and generator excitor systems. Our technicians are trained not only on how to safely conduct testing that will meet applicable codes, but they are also trained and equipped on how to observe component performance, make necessary adjustments and recommend component repairs as required.

Ultimately, the use of load banks has an important role in helping us deliver to you, our customer, a testing solution that not only meets the code requirements also ensures system integrity and reliability, to make sure it works when you need it! After all, isn’t that what you’ve come to expect from your generator service provider? At Austin Generator Service, this is our commitment.

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