Ahmed Elsayed

Wet Stacking of Generator Sets in HealthCare Facilities & How to Avoid it 

Blog Post created by Ahmed Elsayed on Sep 1, 2019


Hospitals are the communities’ life support system in both times of crisis and in times of general healthcare needs. If something like a power outage occurs, vital power is still needed to keep life support machines and other necessary healthcare tools working in order to sustain human life. This is where the importance of generators comes in. When the power is out, a hospital almost always uses a backup generator to make sure that everything remains on and running. The need for generators in the healthcare market is one of absolute importance and a large percentage of standby power systems use the diesel engine. Diesel generator is a convenient independent fuel source and the compression ignition systems of diesel engines have a much higher thermal efficiency than the spark ignition system used by gas engines. However, one factor to be considered when selecting a diesel power source is the potential for “Wet Stacking.”

What Is Wet Stacking?

Wet stacking is a condition that occurs when your unused diesel fuel, accumulated moisture, and carbon particles are allowed to gather in the exhaust of your generator. This can happen for a number of reasons but usually results from diesel fuel not being properly burned off during use. As a result, a dark muddy liquid congeals in the generator’s exhaust stacks — hence the term wet stacking.

What Causes Wet Stacking?

When unburned fuel is exhausted out of the combustion chamber, it starts to build up in the exhaust side of the engine, resulting in fouled injectors and a buildup of carbon on the exhaust valves, turbo charger and exhaust. Excessive deposits can result in a loss of engine performance as gases bypass valve seating’s, exhaust buildup produces back pressure, and deposits on the turbo blades reduces turbo efficiency. Permanent damage will not be incurred over short periods, but over longer periods, deposits will scar and erode key engine surfaces. (Diagram One) Also, when engines run below the designed operational temperature, the piston rings do not expand sufficiently to adequately seal the space between the pistons and the cylinder walls. This results in unburned fuel and gases escaping into the oil pan and diluting the lubricating properties of the oil, leading to premature engine wear.



How to avoid wet stacking?

The best way to alleviate the effects of wet stacking is to apply additional load to the gen-set, increasing operating temperatures to burn off accumulated unburned fuel and carbon. The amount of minimum load varies per engine manufacturer; Also NFPA has guidelines to reduce the effects of wet stacking on backup power systems: NFPA guidelines in Level 1 and 2 applications require exercising the unit, at least monthly, for 30 minutes under either of two methods: (NFPA 110 8.4.2) (NCE Article 517) 1-Loading that maintains the minimum exhaust gas temperatures, as recommended by the manufacturer. 2-2-under operating temperature conditions and at not less than 30 percent of the EPS standby nameplate kW rating. Periodic Preventive Maintenance (PPM) Without proper maintenance generators are more susceptible to fuel problems like algae build up on the fuel system, clogged fuel injectors, wet stacking and dead batteries. These types of problems can even ruin your generator itself if they are let go for too long.

HealthCare Accreditation & Additional Conditions !

JCI-The Joint Commission on Accreditation of Health Care Organizations (JCAHO), the organization that accredits health care institutions, has taken this testing to a level beyond the NFPA. They require testing of 12 times per year with testing intervals between 20-40 days. Testing generators for at least 30 minutes under a dynamic load of 30% or greater of the name plate rating. Systems that do not meet the 30% load capacity have three options. 1. Increase the load to meet or exceed 30% of the name plate rating, 2. Maintain the minimum exhaust temperature as recommended by the engine manufacturer, or 3. Undertake load bank testing for a total of 2 hours continuous loading as follows: a) Load at 25% of name plate for 30 minutes b) 50% for 30 minutes c) 75% for 60 minutes.

CBAHI- central board for accreditation of healthcare institutions(KSA), they require testing this following: FMS.31.2.2 The hospital performs weekly test without load for ten minutes. FMS.31.2.3 The hospital performs monthly on load test for thirty minutes. FMS.31.2.4 The hospital performs full load test every three years on external load. FMS.31.2.5 The hospital generator starts normally without load for ten minutes. ,But without specific percentage load as mentioned in the NFPA 110 that should be 30% load.

NSR- National Safety Requirement Egyptian Healthcare Accreditation, they require inspection documents & preventive maintenance schedule, contracts & equipment, as well as testing results of generators, tanks &/or other key system to make sure of facility coverage 24/7. NSR 30- There is a well-structured & implemented plan for regular inspection, maintenance, testing & repair of essential utilities addressing at least the following: NSR30.1 Electricity; including stand-by generators , But without specific frequency and percentage load of the test as mentioned in the NFPA 110 that should be 30% load for 30 minutes.

Reference Codes and standards

NEC, primarily Articles 445 (Generators), 517 (Health Care Facilities), and 700 (Emergency Systems).

NFPA 99: Health Care Facilities Code.

NFPA 110: Standard for Emergency and Standby Power Systems.

Ahmed ElSayed Abudl Salam, Facility management and safety officer at king saud university medical city 


Email: unjumeau.a@gmail.com