Can Two Diesel Generators Share One RECD?

Every facility manager with two generators has done the same arithmetic at some point. Set A is the main unit. Set B is the standby or the second half of a paired system. Only one usually runs at a time, and a quote for two RECDs is roughly double the quote for one. So the natural thought arrives: “Why not fit a single device and let both sets share it?”

The idea sounds like sensible cost engineering. It is also one of the more expensive mistakes a site can make, because the saving is visible on day one and the problems appear later, under load, in the middle of an outage or during an inspection. This article puts the sharing idea on a test bench. It first explains why the numbers rarely add up, then walks through each of the five states a two-generator site passes through in real life, then lays out what to do instead.

The short answer

Sharing one retrofit emission control device (RECD) between two diesel generators is not recommended. An RECD is sized, tested and approved as a device for one engine, with its own exhaust flow, temperature and back pressure limit. Joining two exhausts into one device creates cross-flow between engines, interacting back pressure, unreliable regeneration conditions, a single point of failure and paperwork that does not match how the device was approved. The standard, safest arrangement is one RECD per engine. Where budget is the concern, there are better ways to reduce cost, and they are covered below.

The spreadsheet temptation

Start with the money, because that is where the idea is born. Below is a purely illustrative comparison, expressed in index units and not rupees, so no one mistakes it for a quote. Let one device for one engine cost 100 units.

Item	Two separate RECDs	One shared RECD
Devices	200	About 150 to 170, since a device for combined flow is larger
Common ducting, isolating dampers and controls	0	About 30 to 60
Engineering, design and integration	Standard	Extra, because the arrangement is custom
Testing and commissioning	Two standard commissioning jobs	One complex commissioning with interaction checks
Risk allowance for problems	Low	Significant
Visible saving on day one	None	Perhaps 0 to 20 units, often nothing

Once you add the ducting, the dampers, the controls and the engineering, the saving shrinks quickly, and it can vanish altogether. The remaining “saving” is then paid for with complexity, and complexity has a cost that shows up over years. For real price ranges by capacity, see our overview of RECD cost in India.

How an RECD is normally matched

To see why sharing is awkward, remember how an RECD is set up in the ordinary case.

One engine, one device. The device is fitted after the engine’s exhaust and, generally, after or in place of the silencer. It is sized for that engine’s exhaust flow and temperature at rated load.

Back pressure is engine specific. Every engine has a limit for how much restriction the exhaust system may impose. As explained in our guide on exhaust back pressure in DG sets, the acceptable value depends on the engine and RECD combination, and there is no universal number. The engineering article by Aurora Generators on exhaust backpressure makes a related point: designers need to assess the combined effect of the whole exhaust path, including pipe size, bends, silencers and after-treatment equipment, not isolated parts.

Approval is granted per capacity band. Type approval is granted after testing against a defined engine capacity, and the testing is done on the device in its ordinary single-engine configuration. Two engines feeding one device is a different configuration from the one that was tested.

The rules speak of sets. The revised schedule for DG sets in the National Capital Region, described in the PIB release on CAQM’s directions, sets requirements by the capacity of each DG set, with dual fuel or emission control device requirements for defined ranges. The natural reading is that each set must be individually compliant, which points to a device per set. Where a facility wants to argue for a different arrangement, that needs to be confirmed with the authority beforehand and not assumed.

With that baseline, we can walk through the five states of a two-generator site.

The run-state walk: five moments a shared RECD has to survive

A shared RECD does not just need to work when everything is calm. It has to work in every state in which the site operates. Here they are one by one.

State 1: Only Set A is running, Set B is at rest

This is the everyday state for most standby arrangements. Set A’s exhaust enters the shared device. But a shared device means a shared duct, and a shared duct connects to Set B’s exhaust outlet as well. Without a proper isolating arrangement, hot exhaust gas can flow backward along the branch towards the stopped engine.

The engineering guidance from Turnkey Industries on generator exhaust systems notes that combined stacks create complex back pressure interactions and require sophisticated design to prevent cross-flow between operating and non-operating units. It explains that motorised dampers are needed to isolate an offline generator, since backflow through a shut-down engine can spin its turbocharger and damage lubrication. In other words, the shared arrangement demands additional hardware, and that hardware has to be reliable for years.

What a good design would need: a damper on each branch, an interlock that closes the offline set’s damper before the online set starts and opens it only when its own set runs, a position feedback signal, and a fail-safe behaviour on power loss. That is a mechanical and controls project, and not a simple fitting job.

State 2: Only Set B is running, Set A is at rest

The same problem, mirrored. Now the damper on A’s branch has to be closed and B’s open. If the interlock has drifted, or a damper is stuck partway from soot, hot gas flows the wrong way. Testing this state periodically means running each set on its own and checking the dampers, which few sites do consistently.

There is also a second issue. The shared device was sized for the combined flow of both sets, or at least for the larger one with margin. When one smaller flow passes through a device sized for more, the exhaust temperature inside the device can fall below the level the catalyst and filter need to clean themselves. Our RECD maintenance guide explains passive and active regeneration. A device that never reaches the temperatures it needs tends to accumulate soot, and rising back pressure follows.

State 3: Both sets run together

Sites that run two sets in parallel, or that start the second set as demand climbs, put both exhaust streams into the same device at once. Now several things happen together.

• Combined flow. The device sees the sum of the two flows, which must fit within its rated capacity. If the sets are unequal, the smaller one may be pushed against the larger one’s exhaust pulses.
• Interacting back pressure. Each engine sees the restriction created by the shared device and duct, not by its own separate path. Whether each engine remains within its own limit depends on the combined system, and measuring that is harder than measuring a single path.
• Uneven loading. If the two sets share load unevenly, their exhaust temperatures differ, and mixing them changes what the device sees.

The problem for the owner is not that this state is impossible to engineer. It is that it is hard to prove that both engines are safe in every load combination, and no standard test for a retrofit device is written around that condition.

State 4: Changeover and start-up

The moments when one set stops and the other starts are the most delicate. During a changeover on an outage, the site is under stress, the ATS is switching, and the dampers must reposition in a defined order. If a start command arrives before the offline branch is isolated, the starting engine pushes cold, sooty exhaust into a duct connected to a stopped engine. If a set trips, and the other set carries on, the tripped set’s branch has to close immediately.

Start-up is also when engines make more smoke, so any hesitation in the damper sequence can send extra soot into the shared device. And when the emergency arrives, the last thing a facility team wants is an exhaust valve sequence that has to work perfectly for the generators to be usable. Our comparison of standby and prime power generators explains how differently these sets are meant to behave, and why simple, independent behaviour is a virtue in standby duty.

State 5: One set is under maintenance

The last state is the most overlooked. Suppose Set B is being serviced while Set A runs. In a separate-device arrangement, Set B’s exhaust system is fully independent and safe to work on. In a shared arrangement, the duct and the device are live with Set A’s exhaust. A technician working on B’s exhaust branch is now exposed to hot gas, carbon monoxide or soot backflow if the damper leaks. To service the shared device itself, both sets have to be shut down, which removes the very redundancy the second set was bought to provide.

The redundancy problem

The second generator at most sites exists for one reason: so that if one set fails, the other carries the load. Engineers call this redundancy. Hospitals, data centres, and manufacturing lines with costly stoppages all rely on it. Our guide on sizing a generator for a hospital touches on why backup capacity is designed in.

A shared RECD reintroduces a single point of failure into a system that was designed to have none. If the shared device chokes with soot, cracks or fails its dampers, both sets are affected at the same time. The site pays a small saving upfront and takes on the risk of losing both machines from one cause. For any facility where downtime is costly, that is a poor trade.

The compliance side of sharing

Even if the engineering could be made to work, the paperwork is a separate question.

• The approval matches a single-engine configuration. A device approved for a defined capacity band on one engine is not automatically approved as a combined system across two engines. Anyone proposing a shared arrangement should be able to show what test or approval covers it.
• Verification is per set. An inspector or consultant checking compliance will typically read the certificate against the set in front of them. With a shared device, which set does the certificate belong to? If the answer is unclear, trouble follows. Our guide on choosing a CPCB-approved RECD vendor explains what a vendor should be able to demonstrate.
• Operating records are per set. Where run-hour logs or restrictions apply, as in the way GRAP restrictions affect businesses using DG sets, each set is treated as a separate source. A device shared between them muddies which set is compliant when.

We have not found a public rule that describes a shared RECD across two engines as an approved arrangement. If someone tells you it is allowed, ask for the document. If they cannot show one, treat it as unproven.

What looks like sharing but is not

There are a few configurations where one device seems to serve two exhausts, and they are worth separating from the true sharing case so you do not confuse them.

One large engine with two exhaust outlets. Some large engines, such as certain V configurations, have two exhaust manifolds. A device with two inlets serves both, but it is still matched to a single engine and has been designed that way. This is single-engine engineering, not sharing.

Modular devices for one large engine. On very large capacities, an RECD may be built from several modules in parallel. Again, the modules serve one engine.

Purpose-built central treatment at a power plant. Large plants sometimes treat the exhaust of several engines centrally, with engineered systems designed from the start for that duty. That is a design decision at plant level, not a retrofit measure that you can apply to two ordinary DG sets on a site.

If your proposal does not match one of these, it is a genuine sharing idea, and the concerns above apply.

Better ways to cut the cost of a two-generator site

Once sharing is off the table, the question becomes how to keep the budget under control. There are several options that do not compromise safety.

Option 1: Standardise and buy as a package

If both sets fall in the same capacity band and have similar exhaust layouts, ask for a multi-unit quote. One site survey, one mobilisation, one commissioning visit and one design can lower the cost per unit. Our guides on the RECD installation timeline and on the RECD installation process describe how multi-generator sites can be sequenced so that each set is done in turn and the site keeps power throughout.

Option 2: Retire one set

If one set is old, oversized or seldom used, ask whether you really need it. Our article on generator load factor and duty cycle helps you look at your true load profile. Sometimes a single correctly sized set, or a smaller standby, is enough, and you avoid spending on a device for a machine you no longer need.

Option 3: Replace one and retrofit the other

If one set is very old, replacing it with a set that already meets current norms may cost less over its life than retrofitting. Compare the paths in our article on RECD versus a new CPCB IV+ genset.

Option 4: Use a dual fuel kit on one of them

Where gas is available, one set can be converted to dual fuel while the other gets an RECD, depending on the regulations for your capacity range. Our comparison of RECD versus dual fuel kit and the dual fuel kit page explain the differences.

Option 5: Phase the work

Retrofit the set that runs the most hours first, and schedule the standby set within a defined period, if your compliance timeline allows it. Do not, however, leave a set that you rely on in an emergency non-compliant without first understanding the local rules.

Option 6: Negotiate one service arrangement

Separate devices do not have to mean separate contracts. A single maintenance agreement across both devices can reduce the cost per visit. See our advice on negotiating an AMC contract for your generator.

Does a standby set need its own RECD if it hardly runs?

This is a very common follow-up. The answer depends on the rules for your capacity range and your location. In the NCR framework described in the PIB release mentioned earlier, requirements are set by the capacity of each DG set and apply to sets that are permitted to operate, including standby sets that start during outages. A set that never runs might be treated differently from one that starts during a power cut, but that is a question for your State Pollution Control Board or consultant, not something to assume. As a working rule, if a set is connected and available to run, plan on it being compliant.

Questions to ask if someone proposes a shared RECD

If a vendor, consultant or contractor suggests sharing, put these questions to them and ask for written answers.

1. Which test or approval covers a device serving two engines?
2. How will each engine’s back pressure be measured and kept within its own limit in every operating state?
3. What isolates the offline engine, and what happens if that isolation fails?
4. How does the device stay at regeneration temperature when only the smaller set runs?
5. What happens to my redundancy if the device or a damper fails?
6. How will an inspector match the certificate to each set?
7. What is the full installed cost including ducting, dampers, controls and engineering, and how does it compare to two devices?
8. Who takes responsibility if an engine is damaged because of the shared arrangement?

A vendor who has thought the arrangement through will have answers. One who has not is offering you an experiment.

A quick decision guide

• Two sets, both needed, and both permitted to run: fit one RECD per set.
• Two sets, one rarely needed: review whether you need both, then decide.
• Two sets, one very old: compare replacing it with a compliant new set.
• Two sets and a tight budget: ask for a multi-unit quote, phase the work if the rules allow, and consider a single maintenance contract.
• Someone proposes a shared device: ask for the approval document and the answers to the eight questions above.

Frequently asked questions

Is it allowed to use one RECD for two DG sets?

We have not found any public guidance that describes it as an approved arrangement. The general framework treats each DG set individually, and RECDs are approved as devices for a single engine’s capacity band. Any proposal to share should be confirmed in writing with your State Pollution Control Board before work begins.

What if both generators never run at the same time?

That helps with the flow question, but the other problems remain. The offline engine still needs isolation, the device sees an off-design flow when only one set runs, maintenance access is harder and redundancy is reduced. Non-simultaneous running does not remove the need for a damper system, and that system is a new source of failure.

Can a bigger single device handle two engines?

A bigger device can handle a bigger flow, but size is only one factor. The approval, the back pressure at each engine, regeneration temperature and isolation issues still need to be solved, and a device sized for a larger combined flow may operate poorly when only one engine is running.

What about two engines in one large genset or a twin-exhaust engine?

That is a single-engine design, and the device is engineered for it from the start. It is different from two independent sets sharing an RECD. If you have such a set, discuss the exhaust configuration with the vendor when you send your engine details. Our guide on DG nameplate photos for RECD enquiries explains what to send.

Is one RECD per engine always the answer for a multi-set site?

For most sites, yes. Where the numbers are difficult, look at retiring a set, replacing one, using dual fuel where suitable, or phasing. Sharing is the option we would try last.

Can I move an RECD from one of my sets to another later?

Sometimes, subject to band, back pressure, fit, paperwork and condition, though it is seldom the cheapest route. Our article on whether an RECD can be transferred to another diesel generator covers the details.

What about generators I rent for a project?

Rented sets come with their own questions about responsibility and modification. See our guide on installing an RECD on a rented diesel generator.

Planning a multi-generator site?

If your site has two or more sets, send us their plates, photographs and a simple sketch of how they are laid out, and we will help you work out the most economical compliant route, whether that is a multi-unit package, a phased plan or a mix of technologies. You can review capacity-specific options on our retrofit emission control device page or reach the team through the contact page.