Your questions answered: Design and installation considerations for generator sets

Responses to questions not answered during the Sept. 13 webcast are answered here.


The webcast focuses on applications, environmental concerns, installation concerns, codes, and standards as they relate to generator sets.

Brian Ponstein, Regional Sales Engineer at MTU Onsite Energy tackled unanswered questions from the Sept. 13, 2017, webcast on design and installation considerations for generator sets.Brian Ponstein, Regional Sales Engineer at MTU Onsite Energy tackled unanswered questions from the Sept. 13, 2017, webcast on design and installation considerations for generator sets.

Question: Are there any standard genset ratings, and for which rating—prime power, emergency standby power?

Brian Ponstein: ISO 8528-1-2005: Reciprocating internal combustion engine driven alternating current generating sets provides ratings that are standard across the industry.

Q: What about limited time prime (LTP) rating?

Ponstein: The LTP rating is a unique rating where the load is constant and the running hours per year are minimal. Many manufactures have ratings to meet these requirements.

Q: Are you talking about natural gas or diesel in the ratings?

Ponstein: The rating of the generator set is independent to the fuel the unit runs on. ISO 8528-1 defines ratings. 

Q: Does the viscosity of the biodiesel affect the generator set durability?

Ponstein: The fuel characteristics required shall meet the engine manufactures requirements. Not meeting the requirement specified by the engine manufacture could result damage to the unit

Q: How does flue back-pressure affect generator capacity and rating?

Ponstein: Exhaust backpressure directly affects the performance of the engine. Excessive backpressure can result in a derating for the generator set. In extreme conditions, it can result in damage of some engine components.

Q: What about exhaust design to accommodate thermal growth? Expansion joints?

Ponstein: Exhaust piping will grow due to thermal expansion. This growth needs to be accounted for in the exhaust system design so that stress is not placed on critical components.

Q: Does MTU do sound testing for its enclosures?

Ponstein: Yes, MTU Onsite Energy performs sound testing for all of its products.

Q: What value should be taken when sizing a generator kVA or kW?

Ponstein: Either is acceptable as long as the power factor is stated as well. If the power factor is not stated then both kW and kVA is needed. Typically, 3-phase units are rated in kW at 0.8 Power Factor for 60 Hz applications.

Q: What are your thoughts on bi-fuel systems for gensets?

Ponstein: Typically, bi-fuel units are a diesel engine for starting with natural gas added to the intake of the unit. This is a modification to the certified state and thus the unit is no longer EPA certified. It is advised to consult the engine manufacturer and regulatory agencies to understand the full impact on a system.

Q: Would gensets for naval installations follow the same standard?

Ponstein: Typically, government applications such as naval have additional requirements that go above and beyond land-based applications.

Q: What are the different generator fuel types used on U.S. federal government generators?

Ponstein: There are generator sets being used for U.S. federal governments that run various types of fuels ranging from gasoline, natural gas, diesel, bunker fuel, jet fuel, and variations of these. Always coordinate manufacture requirements with fuel specifications.

Q: For gensets, are lead acid batteries the only option? Is there any benefit in specifying AGM batteries?

Ponstein: Lead acid batteries are the most common battery type used today in emergency generator set applications. There are many other battery types available and have been used with acceptable performance. It is all about designing the system right and having the proper education onsite about how to maintain the batteries.

Q: You touched on digital controllers for synchronizing. Do the controllers “do it all” or are special breakers and switchgear required?

Ponstein: When paralleling with an MTU Onsite Energy generator controller, there is not a requirement for traditional switchgear controls to be used, but electrically operated circuit breakers are required. The use of switchgear and distribution breakers could be required if the site requires special operation of downstream loads. Please work with your local MTU Onsite Energy distributor for more details.

Q: Do natural gas and diesel have the same EPA restrictions for non-emergency use from a 150kW unit?

Ponstein: No, the EPA separates the natural gas and diesel regulations. Furthermore, each section is split into emergency and non-emergency. The emissions from a diesel unit are different than a gaseous unit. For example, PM is not a regulated emission for gaseous units whereas it is for diesel.

Q: Is digital instrumentation more susceptible to vibration and is analog instrumentation generally better? Or is there no significant difference?

Ponstein: Instrumentation—digital or analog—used by MTU Onsite Energy has been tested and performs as intended for generator set applications.

Q: How would multiple generators be paralleled if each generator's pitch was different?

Ponstein: When paralleling units that do not have the same pitch, an analysis must be performed to verify the effects of harmonics and circulating currents in the system before paralleling units. Often, custom alternators are provided to match the pitch of existing units.

Q: What are the negative effects of operating the generator at less than 30% load?

Ponstein: The exact point for a given unit does vary, but 30% is a guideline value that has been used for many years and is supported by all major manufactures. The effects are oil dilution, cylinder glazing, buildup of unburned hydrocarbons in the exhaust, and white smoke. If after treatment is used, additional load may be required to reach the exhaust temperatures needed for that system.

Q: What is a guideline for sizing gensets for existing buildings? Can one year’s worth of electrical utility bills (kW demand) X 1.25 be sufficient?

Ponstein: MTU Onsite Energy recommends the use of PS Spec for sizing generator sets. When using utility bills, it averages the consumption of power over a period of time. The SkVA or SkW of the load is a critical component in sizing a generator set and this information is missing from the information provided in utility bills.

Q: How important is it to properly size the exhaust duct? What are the impacts?

Ponstein: If the exhaust duct is not sized properly, it can add restriction. Always size the exhaust system to comply with the back-pressure limits of the engine generator set. Back pressure restriction above the manufacturer’s limits can result in performance issues, such as not reaching full power at site conditions.

Q: Can the electric company determine if a backup generator is needed based on a shopping center’s history of power outages?

Ponstein: In this case, it is typically a financial choice of the owner based on the risk for the site. The electrical company could provide information to aid in a risk analysis, but it would be up to the owner to weigh the risk against the cost.

Q: Do you feel that with the proliferation of LED lighting and VFDs as retrofits, the true effect on generators is not being seen because the unit now ends up being oversized?

Ponstein: With LED lights and VFDs, it is becoming easier to load a generator set to higher load factors because the sizing of the generator is not as dependent on motor starting but rather running-load and harmonics. Please use MTU Onsite Energy’s Sizing Software PS SPEC to size units for LED and VFD loads.

Q: What are standard sound parameters for enclosures and silencers, i.e., level 2, critical, hospital, etc.?

Ponstein: When it comes to sound standards for enclosures, there is not a defined industry standard. All ratings are unique to each manufacturer, but a specific sound rating at a specific distance should be specified.

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