Planning Safe Access to RECD Sampling Ports on a Tall Chimney

A stack testing team arrives at a plant at nine in the morning. They have a sampling train, a probe several metres long, a gas analyser, a laptop and a heavy case of glassware. The generator is warmed up and loaded. The sampling ports are exactly where the drawing says they should be: on the chimney, 18 metres above the ground. There is no ladder. There is no platform. Someone suggests a borrowed extension ladder and a rope.

That test does not happen that day, or it happens unsafely. Either outcome is a planning failure, and it is almost always a failure that could have been fixed on paper months earlier, when the chimney was being designed or modified for the retrofit emission control device.

This guide is about that planning. It works from the bottom of the chimney to the top, the way a technician actually travels, and looks at each stage of the journey: the approach on the ground, the climb, the platform, the port itself and the work that happens there on test day.

Why the Sampling Port Matters After an RECD Retrofit

An RECD is fitted to bring particulate emissions down. Showing that it does so, at commissioning and over the years that follow, usually means measuring what comes out of the stack. The CPCB System and Procedure for Emission Compliance Testing of RECDs governs how devices are approved, and site-level measurements are often requested by State Pollution Control Boards, consent conditions, clients or auditors. Our guide on whether a smokeless DG set still needs particulate measurement explains why a clean-looking plume is not the same as a measured result.

A measurement needs a port. A port needs a person beside it with equipment. A person beside a port 15 or 25 metres up needs a safe way to get there, a safe place to stand and a safe way to bring the equipment up and down. If any of those is missing, the port is decorative.

Why tall chimneys are common on DG sets

Indian rules link DG stack height to the set’s rating and the building it sits beside. The widely used formula for DG sets is that stack height above ground equals the height of the building plus 0.2 times the square root of the set’s kVA rating, with minimum values for smaller sets. State boards publish their own versions; the Haryana State Pollution Control Board note on DG stack height is one example. The result is that a 1000 kVA set beside a four-storey block can need a stack top around 20 metres above ground. Our article on whether increasing chimney height is an alternative to an RECD explains why height helps dispersion but is not a substitute for treatment.

So the tall chimney is often a given. The question is how to make the sampling point on it reachable.

Stage 0: Decide Where the Port Must Be Before Deciding How to Reach It

Access planning starts with port position, because the position sets the height of every ladder and platform that follows.

The flow-disturbance rule

Particulate sampling works best where the gas flow is straight and even across the pipe. Bends, reducers, dampers, silencers and the RECD outlet all disturb the flow. Stack testing methods therefore ask for the port to sit a certain number of pipe diameters downstream of the last disturbance and a certain number upstream of the next one, or of the outlet. A common reference is the US EPA’s Method 1 for sample and velocity traverses, which uses about eight diameters downstream and two diameters upstream as the preferred position, with shorter distances allowed only if more traverse points are measured. Indian stack monitoring practice follows a similar logic. The accredited laboratory that will test your stack, and your SPCB consent conditions, decide what applies to you.

Turning diameters into metres

Stack internal diameter	8 diameters downstream	2 diameters upstream	Minimum straight length for the preferred position
150 mm	1.2 m	0.3 m	about 1.5 m
250 mm	2.0 m	0.5 m	about 2.5 m
350 mm	2.8 m	0.7 m	about 3.5 m
450 mm	3.6 m	0.9 m	about 4.5 m

These numbers are illustrative. They show something useful: on a DG chimney, the straight length needed is often only a few metres. That means there is frequently a choice about where on the stack the port goes, and that choice decides how hard access will be.

Look for the lowest compliant position

On a tall chimney that rises straight from a horizontal run, the first bend at the foot of the riser is often the last disturbance. Eight diameters above that bend may be only two or three metres up. Placing the port there, rather than near the top, can turn a 20 metre climb into a short ladder or even a ground-level platform. Ask:

• Where is the last bend, reducer or fitting before the outlet?
• How many straight diameters are there above it?
• Is there a compliant position low enough to reach from the ground, a roof or an existing floor?

If the chimney passes a roof or a terrace on its way up, a port just above that level, with the required straight length below it, can often be reached from the roof itself.

The RECD’s own outlet counts as a disturbance

The RECD outlet cone is a change in section. If the device sits at the foot of the chimney, the port needs its straight length above the device outlet, not above the engine. Our guide on horizontal vs vertical RECD mounting shows how orientation affects the straight run that follows.

Stage 1: The Approach on the Ground

Before anyone climbs, they have to reach the base of the chimney with their equipment.

Questions for the ground stage

• Route. Is there a clear, lit route from the vehicle drop-off point to the chimney base, without crossing live roadways, cable trenches or wet floors?
• Laydown area. Is there a flat, dry space near the base, of a few square metres, where the team can unpack and assemble equipment?
• Overhead hazards. Are there overhead cables, pipe racks or the DG exhaust outlet itself near the climbing route?
• Exclusion zone. Can the area under the platform be barricaded during testing, so that a dropped tool does not hit anyone?
• Exhaust and heat. Is the base near hot exhaust, radiators or other DG sets that will be running during the test?

Where the chimney is on a roof

Rooftop chimneys add a second approach stage: getting people and equipment onto the roof. Check the roof access route, the load capacity of any roof walkway, edge protection along the route and whether the roof is fragile. Our guide on rooftop RECD installation covers roof access and lifting.

Stage 2: The Climb

There are several ways to reach a sampling port high on a stack. Each suits different heights, frequencies and budgets.

The access options compared

Option	Suits	Strengths	Limitations
Fixed stair with landings	Ports tested often; heavy equipment; sites with many visitors	Easiest and safest to use; equipment can be carried by hand	Highest cost; needs space and structure
Fixed ladder with fall-arrest system or safety cage	Most DG chimneys of moderate height	Moderate cost; small footprint	Hands busy during climb; equipment must be hoisted
Fixed ladder with rest platforms	Tall chimneys	Breaks long climbs into stages	More steel; each platform needs guardrails
Mobile elevating work platform (boom or scissor lift)	Occasional tests; good ground access	No permanent steel on the stack	Needs hard ground, space and a trained operator; hire logistics on test day
Scaffold erected for each test	Rare tests; awkward geometry	Flexible	Time, cost and inspection every time
Port relocated to a reachable level	Where flow rules allow	Removes the access problem	Only possible if a compliant position exists

For a DG chimney tested once or twice a year, a fixed ladder with fall protection and a proper platform at port level is the most common good answer. Mobile platforms are a reasonable alternative where ground access is open and level, but the plan must make sure one can actually reach the port on the day.

Fixed ladder details worth specifying

• Fall protection. A fixed ladder above a certain height needs either a cage or, increasingly preferred, a vertical lifeline or rigid rail with a fall-arrest device. The general principles are described in the fall protection article. Your site safety rules and applicable regulations decide which is required.
• Rest platforms. Long climbs should be broken by intermediate landings.
• Clearance from the hot pipe. The ladder should not sit where climbers brush against the chimney. Leave a gap and consider a guard over the hottest section.
• Exit at the top. The climber should step off the ladder onto the platform through a self-closing gate, not climb over a guardrail.
• Corrosion protection. Ladders on chimneys see heat, soot and weather. Galvanising or a suitable coating, and stainless fixings in coastal air, extend life. Our guide on choosing an RECD for coastal salt-laden environments covers material choices near the sea.
• Inspection. Ladders and fall-arrest systems need periodic inspection and records, like any other safety equipment.
• Capacity. Testing is rarely a one-person job, so size the ladder system and platform for at least two people plus equipment.

Stage 3: The Platform

The platform is where the work happens. A poorly planned one forces technicians to lean, stretch and improvise with heavy equipment near a hot pipe.

Platform checklist

Size and shape

• Enough depth in front of the ports for a person to stand and handle a probe
• Enough width for two people and equipment cases
• On a round stack, a part-circle or full-circle platform if ports are at 90 degrees to each other

The probe problem

The sampling probe is pushed across the stack diameter through the port. That means there must be clear space outside the port at least as long as the probe, in line with the port. On a 450 mm stack, the probe and its handle can need well over a metre of straight clearance behind the port. If a guardrail, a wall or another pipe sits in that line, the probe cannot be inserted. Ask the testing laboratory for their probe length and check the line on the drawing.

Edges and guarding

• Guardrails at top and mid height on all open sides
• Toe boards to stop tools and fittings rolling off
• A self-closing gate at the ladder entry
• Mesh or solid flooring, with mesh openings small enough that small parts do not fall through

Heat and exhaust

• A guard or insulation on the chimney where people stand close to it
• Platform position far enough below the outlet that the plume does not wash over the team
• Wind direction considered: a port on the downwind side of the outlet gets the plume blown back down in some conditions

Load

• Rated for people, equipment and any small hoist or davit used to lift equipment
• Structural connection to the chimney or to a separate frame, designed by an engineer

Our article on whether an RECD can replace a generator’s exhaust silencer touches on how changes to the exhaust can alter chimney loads; any platform added to a chimney adds load too.

Lifting equipment to the platform

People should climb with their hands free. Equipment comes up separately.

• A small davit or gin wheel at the platform, with a rope and bag, for light loads
• A rated hoist for heavier cases, with a clear lifting zone below
• A tag line to stop loads swinging into the ladder or chimney

The lifting zone below must be inside the exclusion zone.

Stage 4: The Port Itself

The port is a small fitting, but its details decide whether testing goes smoothly.

Port hardware

Detail	Good practice
Number of ports	Usually two at 90 degrees to each other on a round stack, so the team can traverse two diameters; confirm with the laboratory
Port size	Large enough for the laboratory’s probe and nozzle; many teams work with ports in the region of 75 to 100 mm nominal, but ask first
Nozzle length	Short enough that the probe can be inserted fully; not projecting inside the stack
Cap or blind	Threaded cap or bolted blind flange that can be removed by hand tools, with a gasket suited to exhaust temperature
Height above platform	Around chest to waist height for the person standing, so the probe can be handled comfortably
Material	Compatible with the chimney; stainless where corrosion is a concern
Insulation	Removable jacket around the port if the chimney is insulated
Labelling	Port identification and orientation marked on the chimney and the drawing

Caps that come off

A port cap that has been heated and cooled for a year, in rain and soot, can seize solid. A team that has climbed 20 metres and cannot remove a cap goes home. Apply a high-temperature anti-seize compound, open the cap at every service visit and replace damaged threads early. Add this to the RECD maintenance guide tasks.

Ports and insulation

If the chimney is insulated, the port nozzle must project through the insulation with a sealed collar. Otherwise rain runs down the nozzle into the insulation, and the cladding around the port becomes the first place that corrodes.

Stage 5: The Work at the Port

Planning access is not only about steel. It is also about what the team needs once they are standing there.

Power at the platform

Isokinetic sampling trains include pumps, heated probes, heated filter boxes and meters. Many need mains power for the duration of the test. Options:

• A weatherproof socket outlet at or near the platform, fed from a protected circuit with residual current protection
• A cable run up from the base for each test, which then becomes a trip hazard and a load on the ladder

A permanent outlet is cheap at construction time and expensive to add later. Our companion guide on the power supply needs of RECD controllers and heaters covers electrical planning around the device.

Stable running conditions and communication

The test needs the set at a defined, steady load, so plan how that load will be provided: plant load, a load bank or both. Give the team a radio link to the DG operator, who must not change load or stop the set without telling them. Our articles on DG load logs for RECD suitability and on generator load factor and duty cycle help when deciding what load to test at.

Stage 6: Permits, People and the Test-Day Plan

A safe access design is half the job. The other half is how the day is run.

Before test day

• Agree the test date with the laboratory, the DG operator and the site safety officer
• Confirm the laboratory’s probe length, port size and power needs
• Inspect the ladder, fall-arrest system and platform; record the inspection
• Open and re-close the port caps to check they move
• Check weather forecasts; high wind and lightning stop work at height
• Arrange the load bank if needed

On the day

Time	Activity	Who
Arrival	Site induction; permit to work for work at height	Site safety officer, laboratory team
First hour	Barricade the exclusion zone; set up equipment at the base	Laboratory team, site staff
Next	Lift equipment to the platform; climb with fall protection	Laboratory team
Next	Start the DG; bring it to test load and stable temperature	DG operator
Test period	Traverse, sample and record; operator holds load	Laboratory team, DG operator
After sampling	Remove probe; refit port caps; lower equipment	Laboratory team
Close-out	Remove barricades; close permit; note any access defects	Site safety officer

Rescue plan

If a person falls and is held by a fall-arrest harness, they need to be brought down promptly. Suspension in a harness for an extended time is dangerous. The site should have a written rescue plan for work at height on the chimney, with the equipment and trained people to carry it out. This is often the most overlooked item on DG sites.

Our article on what happens during a pollution control board inspection explains how inspectors approach testing, and the environmental compliance checklist for generator owners is a good place to record test dates and results.

Special Situations

Several DG sets, one chimney or a cluster of chimneys

Sites with several sets often run their stacks side by side or combine them in a shared structure. Each stack usually needs its own ports. A shared platform can serve several stacks if it is designed for the probe clearance of each. Our article on whether two diesel generators can share one RECD discusses combined arrangements.

Sets with multiple exhaust outlets

Large engines with two exhaust banks may have two pipes rising side by side, each with its own RECD. Each needs its own ports, and the platform must give probe clearance for both. Our guide on RECD fitment for DG sets with multiple exhaust outlets covers the device side.

A Walkthrough: Specialty Chemicals Plant in Pune

This is an illustrative case.

The site

• A 1010 kVA set in a generator building beside a four-storey process block
• The DG stack was required to reach about 22 metres above ground
• The RECD sat at the foot of the stack, horizontally, followed by a 90 degree bend into a 350 mm diameter vertical chimney
• The original drawing showed two sampling ports at 19 metres, chosen by placing them “near the top, clear of the bend”
• No ladder or platform was shown

The review

The plant’s safety officer refused to sign off the drawing without an access plan. A short meeting brought together the RECD vendor, the stack testing laboratory and the structural consultant.

Question	Finding
Where is the last disturbance?	The bend at the foot of the chimney, 2.4 m above ground
What straight length does the method need?	About 8 diameters downstream, roughly 2.8 m on a 350 mm stack
Is a lower position possible?	Yes. The chimney passed the generator building roof at 6.5 m, more than 4 m above the bend, giving more than 11 diameters of straight run below it
Is the roof accessible?	Yes, by an internal stair, with a parapet on three sides
What does the laboratory need?	Two ports at 90 degrees; probe clearance of about 1.6 m; a 230 V socket

The redesign

1. Ports moved from 19 metres to about 7.6 metres, a little above the generator building roof level
2. A small steel platform built off the roof slab around the chimney, with guardrails, toe boards and a gate
3. Probe clearance of 1.8 m kept clear in line with both ports by turning one port to face along the roof
4. A weatherproof 230 V socket with residual current protection fixed at the platform
5. A removable insulation jacket and anti-seize caps on both ports
6. Exclusion zone below the roof edge marked on the drawing for test days

The climb dropped from 19 metres on an unbuilt ladder to a roof stair and a short step up. The first commissioning test took half a day rather than the full day the laboratory had planned. Chemical sites also carry hazardous area and permit considerations; our guide on RECDs for pharmaceutical plants and change control covers documentation in regulated plants. The 1010 kVA DG set RECD page shows the rating used here.

The Mistakes That Stop Test Days

The same handful of errors cancel more stack tests than anything else: ports placed high by default, no probe clearance, a platform too small for two people, no power at the top, caps seized solid, a platform sitting in the plume and no rescue plan. Each one is cheap to avoid on the drawing and expensive to fix on a finished chimney. Several appear on our wider list of common RECD installation mistakes.

Questions to Ask Before the Chimney Is Fabricated

1. Which laboratory will test the stack, and what port size, number and position do they need? On an older chimney, are the existing ports still usable after the retrofit?
2. Where is the last flow disturbance, and what is the lowest compliant port position?
3. Can that position be reached from an existing roof, terrace or floor?
4. If not, what is the access method: stair, ladder with fall protection, mobile platform or scaffold?
5. What platform size and probe clearance are needed?
6. Where will power for the sampling train come from?
7. How will equipment be lifted to the platform?
8. What is the rescue plan for work at height?
9. Who inspects the ladder, platform and fall-arrest system, and how often?

These questions belong in the RECD installation process before fabrication, and the answers are a good measure when choosing CPCB approved RECD vendors.

Frequently Asked Questions

Where should a sampling port be on a DG chimney?

In a straight section of stack, a set number of diameters downstream of the last bend or fitting and upstream of the outlet. Methods such as EPA Method 1 prefer about eight diameters downstream and two upstream. Your testing laboratory and SPCB conditions decide what applies.

Does the sampling port have to be near the top of the chimney?

No. It has to be in a section with suitable straight flow. On many DG chimneys that section starts only a few metres above the base, which can make access much easier.

Is a ladder enough to reach a sampling port?

A ladder alone is not a workplace. Technicians need a platform with guardrails, space for a probe and somewhere to put equipment. The ladder needs fall protection.

Can a mobile lift be used instead of a fixed platform?

Yes, if the ground is suitable, the lift can reach the port and a trained operator is available. It suits sites tested rarely. Plan its availability for each test.

Who is responsible for safe access on test day?

The site owner usually provides safe access and controls the permit to work. The testing laboratory works within that system and follows its own safety rules.

Make the Port Reachable Before You Make It Compliant

A sampling port is only useful if someone can stand beside it safely with the right equipment for a few hours. That outcome depends on a handful of early decisions: put the port at the lowest position the method allows, plan the climb, give the platform room for people and probes, put power and light at the top, protect the area below and write down how the day will run. Make those decisions before the chimney is fabricated and the stack test becomes a routine visit rather than an argument at the base of a 20 metre pipe.

Aceget reviews sampling port positions and access as part of every RECD layout and coordinates with stack testing laboratories before fabrication. To plan safe access on your chimney, contact the Aceget team, or browse the retrofit emission control device range.