Gas engine driven compressor

[paulpaul]

Hi to all

I am a designer in a medium-size italian CNG recip compressor manufacturer company.
We are compressor manufacturer and packager at the same time: all our compressor (till now) have been driven by small low voltage electric motors (400V 37-400 kW).
Now we decided to offer our customers also the gas engine option: having little or no experience in this field, we contacted some of the primary world gas engine manufacturers.
Despite this, I have a basic question about hazardous area classification in case of compressor + gas engine skids.

If we assume the compressor contained in an enclosure, with given ventilation conditions inside and a certain number of gas emission sources, the compressor itself (seen as a generic gas plant composed of potentially leaking components) generates an hazardous area (e.g. zone 1 having done calculations accordingly to EN60079-10).
Now, if we use an electric motor as a driver, we shall use a proper protection mode to fulfill ATEX requirements (e.g. EEx-d motor).
My question now is: if we use a gas-engine, which - I think - does not carry any ATEX certifications suitable for use in zone 1 (except probably for the instrumentation circuit, which could be EEx-i), should I put compressor and gas engine in two separate rooms? Or can I put them in the same room but loosing ATEX certification of the whole package?
I know that in process plant applications (we made some fuel gas and small pipeline compressors but always EM driven) gas engine and compressor are adjiacent, but I know also that someone else uses the two rooms solution, with the shaft passing through the separating wall by means of a gas-tight bearing group.

In general, I will be very grateful if you can adress me to any links you may know about safety issues (explosion hazards and solutions) related to gas engines/compressor packages.
Thx in advance,

 

[zdas04]

This is a tough problem that many people are skirting in many parts of the world. One caution though, if you go with the two rooms approach you are far better to put a genset in one room than to try to run a shaft through the wall. Running a shaft through the wall can work, but gross alignment in conjunction with foundation settling can be an impossible situation.

With regards to your basic question, good luck and tell us how you solve this (none of my clients has been totally successful to this date).

David

 

[ARenko]

If you had enough ventilation could the area be considered Zone 2? And if Zone 2 can you get the engine in a Zone 2 rating?

I assume you're talking about a natural gas engine. We normally work with diesel engines and can get these certified as components to ATEX Zone 2. We then add explosion protection equipment (exhaust gas cooling, flame traps, inlet shutdown valve, etc...) so the entire engine installation is suitable for Zone 2 and in accordance with BS EN 1834.

 

[zdas04]

Up until the current version of API 521, you could change a Zone 1 to a Zone 2 with ventilation, they closed that loop hole in the latest version.

David

 

[paulpaul]

Thank you David

I was thinking for a long time that the problem was quite tough under this aspect.
Anyway, if I decide to go with the two rooms approach I will consider both gensets and also cardanic shafts in the effort to minimize alignement issues.
I must admit that I prefer the one-skid solution, without any wall between the two machines.
Paolo

 

[zdas04]

API 520 and 521 have a very interesting exclusion--gensets are explicitly excluded from the requirements. That makes the genset/electric-driven compressor idea even more attractive.

David

 

[dcasto]

What is your ignition source from the engine? All newer engines have Class 1 Div 1 ignition systems, so where is the issue? With enough ventiation and gas detection you have back ups. Then where is the leak point from the ignition? If you have the packing vented away, another issure solved.

There are millions of gas engine/gas turbine packages through out the world without walls and seals. Go ask your competitor nuevo pignone.

 

[paulpaul]

dcasto,
I agree with you that reasonably, with enough ventilation, protected ignition system and (I add) insulated exhausts plus protected (EEx-i for example) instrumentation on the engine itself, there is no need of any walls or seals. I know that most of gas engine/compressor packages have nothing to do with walls and similar stuff.
The issue is that if we want to certify CE the whole package (and this is very often requested by our clients and mandatory in EU) we have to fulfill all applicable EU directives. Among the others, ATEX is the key one in this case.

As you know, ATEX requires that you put the "correct" devices (electrical or not) in a previously classified area. This means that first you have to classify the room containing the equipment (accordingly for example to std EN60079-10), then choose the right protection mode for the equipment you need to install inside (could be a simple pneumatic actuator or an electric motor).
Now, since a compressor package is made by a number of possibly leaking points (fittings, flanges, etc) that, accordingly to the std, you must consider, you must de-classify the zone with adeguate ventilation. Making the calculation, the result is normally a Zone 1 all around the compressor package.
If the gas engine is adjiacent to compressor w/o any separation, it must comply with this classification (as, for example, an electric motor with EEx-d protection does) in order to get the approval for the whole package. I don't know the exhistance of such an engine. Even GE Nuovo Pignone (I know that factory quite well) made some CNG compressors with the wall solution just because ATEX certification problems. I saw also some Ariel compressors (I don't remember the packager) in which even the electric motor was outside compressor room just because they used a non-ATEX motor.
The toughness of this problem was confirmed also by well known gas engine manufacturer I have contacted here.

 

[dcasto]

You have to ask the question again, what is the ignition source on a sparkfired gas engine? The only ignition source would be an open spark plug and there are none.

In your scenerio, how could you ever build a gas fired direct heater? There is a open flame and you have flages and valve seals near the heater, so no leak points near a heater, even a boiler has fuel going to it, how do you have fuel lines going to the heater?

At least in a gas engine, the fire is contained in a vessel we call a cylinder. The exhaust headered operates at tempers below ingnition temps of gases, the spark plugs and coils are all enclosed. Noone makes intergals any more because of cost, but how could build one? How can you bring natural gas fuel to an engine, even if the engine is behind a wall, it still needs fuel.

 

[ARenko]

dcasto,
I don't know nat gas engines, but speaking of diesel installations we address the following ignition sources after buying an engine certified f/ Zone 2 as a component (i.e. everything that comes w/ the engine is hazardous area already):

engine surface temp
exhaust pipe surface temp
exhaust gas temp
alternator
hot carbon particles exiting the exhaust
flames exiting the inlet or outlet due to ingestion of flammable gas
batteries
sparks from fan/ shroud
sparks from statically charged belts

I've always wondered about nat gas engines. If I understand you correct the surface and exhaust temps are non-issues, correct? What is a typical high exhaust temp or engine surface temp?

paulpaul,
You say you must de-classify the area with adequate ventilation, but that you calculate Zone 1 area. Does this mean you can't adequately ventilate? Or what do you mean by de-classify and adequate? In API 505, if a compressor is in an enclosed, adequately ventilated area it is Zone 2. Are you sure this is not a case of a risk assessment gone too far?

 

[dcasto]

natural gas engines can have exhaust temps into the turbo charger at 1300 F, skin temps on the turbo charger are below 1000F. Everything else is cooler than 400F.

You can use a water cooled exhaust without a turbo charger and get skin temps of less than about 800F.

The alternator, coils and spark systems are all Class 1 div 2 rated with CSA approved stamp or equal.

there are no hot carbons when using natural gas.

Batteries are sealed type, but they are to back up the PLC that monitors the compressor and are not used with the engine per se.

One vendor wanted to sell us sparkless belts, so they must exist.

 

[paulpaul]

dcasto and djv
if we push the analysis further, also a roller bearing, a flexible coupling and (as you note) belts could be ignition sources.
CSA certified devices or systems, from what I know, are not considered in ATEX directive.
The risk of going too far with the risk assessment and to produce a lot of paper rather than compressors is particularly high.
In your example, gas line for the engine is at very low pressure, allowing to consider very small "clouds" of classified area: with "proper" ventilation grade, the standard (I know EN60079-10 that is widely used here in Europe) tells you that you can even consider a typical engine room as a non-classified area. Howewer, you'll want to add gas sensing equipment and block&vent equipments, intrinsic protection instrumentation on the engine (EEx-i), etc. to be even safer. The problems could arise when you put near the engine a compressor in wich pressures could be quite higher than engine inlet gas line (200-300 bar in CNG service) and area classification doesn not help you very much.

 

[ARenko]

Thanks dcasto. I guess those temps keep you below auto ignition temp, but we have to meet T3 temperature class meaning we need to keep surface temps below 200C. We go by BS EN 1834 - Reciprocating internal combustion engines. Safety requirements for design and construction of engines for use in potentially explosive atmospheres.

 

[gurmeet2003]

I do not understand the difficulty in this case. I would have thought that a natural gas engine would be a natural driver for a natural gas compressor. We have sold tens of thousands of skid mounted natural gas compressors driven by natural gas engines. They are either mounted on a common skid, either outdoors or in compressor room. Two room solution has never been used.

Gurmeet

 

[dcasto]

gurmeet2003, there are still people in the chemical industry that could not believe that I wanted to take their excess ethane at 250 psig and compress it to 1000 psig with a gas engine package. They elected instead to install a refrigeration system to condense it with C3= and pump it to 900 psig. They said the gas driven engine would be too dangerous and expensive.