What does the criticality worksheet do?
The worksheet turns a vague statement such as “air is critical to the plant” into a set of specific, testable service requirements. For each air user or group of users it records the process function, the consequence of losing air, how quickly that consequence arrives, the service needed and the recovery route. It then gives a priority score to help decide where standby capacity, storage, spares or rental cover will be worth the investment.
The score is a planning aid. It does not certify safety, prove availability or create a guarantee from any supplier. The value lies in the written reasoning behind each score, which operations, maintenance and management can all check.
Start with the service, not the equipment
Write a functional statement for each user, in language operations can verify. For example: “maintain at least 6 bar(g) (87 psig) and the specified dew point at the packaging-line valve island while one compressor is out of service”. Then record:
- What the air does at that point: actuation, instrument signal, product contact, cooling, blowing, conveying or tools.
- Where the requirement is measured: at the user, not at the compressor discharge gauge.
- The minimum pressure, flow and air-quality class the user needs.
- What happens as supply deteriorates: alarm, slow-down, scrap, unsafe state or shutdown.
- The minimum service needed for a controlled, safe shutdown if normal production cannot continue.
Keep one record per materially different consequence. Workshop tools, control valves and product-contact blow-off may share one compressor station, but their tolerable interruption, quality needs and recovery routes are very different. A single plant-wide score hides those differences.
Separate consequence from likelihood
Consequence asks how bad a loss of air would be. Likelihood asks how often the supply path is expected to fail. Mixing them leads to poor decisions: a highly reliable compressor does not make a severe consequence acceptable, and a frequent nuisance trip on a low-consequence user does not justify a second machine.
| Consequence level | Typical description | Example users |
|---|---|---|
| 5 – Severe | Safety, environmental or regulatory impact, or loss of a complete batch | Safety-related actuators, sterile process air, instrument air for burners |
| 4 – Major | Whole line or plant stops; long restart | Main process air header, PET blowing, kiln or furnace controls |
| 3 – Significant | One line stops or quality falls; product at risk | Packaging lines, pneumatic conveying |
| 2 – Moderate | Slower work or rework; no lasting damage | Cleaning, blow-off, assembly tools |
| 1 – Minor | Inconvenience only | Workshop and general-purpose outlets |
Pair the consequence level with the time to consequence: how many seconds or minutes the user can ride through on stored air before the effect begins. That figure decides whether storage, an automatic standby start or a rental machine is the right protection.
How to fill in the worksheet
- List air users and group them by consequence and supply path.
- For each group, state the required pressure, flow, air quality and continuity.
- Assign a consequence level (1–5) and record the time to consequence.
- Assess the likelihood of loss for each element in the supply path: compressors, dryers, filters, drains, piping, controls and power supply.
- Identify single points of failure, such as one dryer serving all compressors or one main header with no isolation.
- Score priority as consequence × likelihood, with a rule that any severe consequence with a single point of failure is flagged regardless of score.
- For each flagged item, record the chosen protection and how it will be tested.
Worked example
A plant has two 37 kW screw compressors, each able to supply about 6 m³/min (212 cfm) at 7 bar(g) (102 psig), one refrigerated dryer and a 3 m³ receiver. Demand is about 9 m³/min on a normal shift. The figures are illustrative. All groups share one supply path, so likelihood is equal.
| User group | Consequence | Time to consequence | Likelihood (1–5) | Score | Finding |
|---|---|---|---|---|---|
| Instrument air to process valves | 5 | Under 1 minute | 3 | 15 | Single dryer is a single point of failure; flagged |
| Packaging lines | 3 | A few minutes | 3 | 9 | One compressor cannot carry full demand |
| Cleaning and blow-off | 2 | Immediate but tolerable | 3 | 6 | Can be shed during an outage |
| Workshop | 1 | Tolerable | 3 | 3 | No action |
Two compressors look like redundancy, but with 9 m³/min demand and 6 m³/min per machine, losing one compressor leaves a 3 m³/min shortfall. The plan therefore needs load shedding (closing cleaning and workshop branches automatically or by procedure), a dryer bypass with its own filtration or a second dryer for the instrument-air group, and a documented route to bring in a rental compressor if a long repair is expected.
Storage also needs a check. Usable stored air ≈ receiver volume × (pressure drop allowed) ÷ atmospheric pressure. With 3 m³ and an allowed fall from 7.0 to 6.0 bar(g), that is about 3 × 1.0 ÷ 1.013 ≈ 3 m³ of free air, roughly 20 seconds of a 9 m³/min demand if all air is lost. If one compressor trips and the other keeps delivering 6 m³/min, the receiver only has to cover the 3 m³/min shortfall, which gives about 59 seconds (2.96 m³ ÷ 3 m³/min). Storage buys time for a standby start; it does not replace a missing machine.
What does N+1 really mean for compressed air?
N+1 means the plant can meet the required demand with any one unit out of service. It must be tested element by element, not counted by machines.
- Compressors: the remaining machines must deliver the critical demand at the required pressure, not just “have some capacity”.
- Treatment: dryers and filters need their own redundancy or bypass routes that still protect critical users.
- Controls: a single master controller or pressure transmitter failure should not stop all compressors.
- Power: a common incomer or panel can take out every machine at once; consider DG backup for critical air.
- Distribution: a ring main with isolation valves allows sections to be repaired without stopping critical users.
- Recovery: spare parts for the most likely failures, and a written plan for bringing in a hire machine with hoses and connections ready.
Setting the service level
The service level is the agreed promise to the process, for example: “critical instrument air maintained at not less than 6 bar(g) and the specified dew point for all single failures; non-critical users may be shed for up to four hours”. Write it down, agree it with production, and test it at least once with a planned shutdown of one compressor. Many plants find their first test reveals a stuck auto-start, a dryer that cannot be bypassed, or a drain that floods when flow changes.
| Protection option | What it covers | What it does not cover |
|---|---|---|
| Standby compressor with auto-start | Compressor trip or failure | Dryer, power or header failure |
| Larger receiver near critical users | Short gaps while standby starts | Any sustained shortfall |
| Second dryer or filtered bypass | Dryer failure or service | Compressor shortfall |
| Rental machine plan | Long repairs, overhauls, shutdowns | The first minutes of an unplanned outage |
| Critical spares on site | Shorter repair time for common failures | Major airend or motor failure |
| Service contract with planned maintenance | Lower likelihood of failure | Consequence if failure still occurs |
Indian plant context
High summer ambient and dust in Telangana and Andhra Pradesh raise the likelihood of high-temperature trips and cooler fouling exactly when production is often at its peak. Power quality problems such as voltage dips and frequent changeovers to generator supply can trip several machines together. Pharma and food plants in Hyderabad, Genome Valley and the industrial estates of Andhra Pradesh usually need particular attention to air quality continuity, because a dryer bypass that keeps pressure up may still breach the required class.
How Reddy Compressors helps
Reddy Compressors, based at Tarnaka, Secunderabad, since 2002, works with plants on the practical side of air continuity. We have supplied compressors on breakdown to leading pharmaceutical manufacturers in Hyderabad and Telangana, and we supply new and used machines, long-term rental and breakdown cover across sectors.
- Site reviews of the supply path: compressors, dryers, drains, piping, controls and single points of failure.
- Rental compressors from our own fleet (50–2,000 cfm, 2–30 bar, diesel and electric, oil-injected and oil-free), with hoses and connection planning.
- Service and AMC for ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic and other brands, as an independent multi-brand specialist, not an authorised dealer. We have tie-ups with all major and reputed brands.
- Spare parts identified from the machine nameplate and confirmed before quoting, to build a sensible critical-spares list.
- Generator support through Reddy Generators, under the same management, where power backup is part of the plan.
Service teams cover all of Telangana and Andhra Pradesh; equipment, rentals and spares are supplied across India.
What to send us
- A list of compressors and dryers with make, model and nameplate photos.
- A simple sketch of the compressor room and main headers.
- Your critical air users and what happens if they lose air.
- Typical and peak demand, or running and loading hours from the controllers.
- Any recent breakdowns and how long they lasted.
Frequently asked questions
How do I decide which compressed air users are critical?
Ask what happens at each user if air pressure, flow or quality is lost, and how quickly. Users whose failure causes a safety, environmental or regulatory impact, or loss of a complete batch, are the most critical. Record the time each can ride through on stored air, because that decides whether storage, automatic standby or rental cover is the right protection.
Is one standby compressor enough for N+1?
Only if the remaining machines can carry the critical demand at the required pressure with the largest unit out of service, and only if dryers, controls, power supply and piping also have no single point of failure. Many plants with a standby compressor still rely on one dryer or one controller, which leaves the air supply exposed despite the extra machine.
How long will my air receiver keep the plant running if a compressor trips?
Usable free air is roughly receiver volume multiplied by the allowed pressure fall, divided by atmospheric pressure. A 3 m³ receiver falling from 7 to 6 bar(g) holds about 3 m³ of free air, which lasts only about 20 seconds at 9 m³/min demand if all air is lost, or about 1 minute if the other compressor keeps running and only a 3 m³/min shortfall has to be covered. Receivers bridge a standby start; they cannot replace a missing compressor.
When should we arrange a rental compressor for breakdown cover?
Plan it before it is needed. Identify the flow and pressure the critical users need, where a hire machine could stand, how it would connect and what power or fuel it needs. With that information agreed, a rental machine can be brought in quickly when a major repair, overhaul or shutdown would otherwise stop production.
Does a criticality score prove our air supply is reliable?
No. The score helps compare priorities and justify spending. Reliability is proved only by testing the plan, for example by deliberately taking one compressor or dryer offline under controlled conditions and checking that pressure and air quality at critical users stay within the agreed service level.
What spare parts should be kept for critical compressors?
Keep the parts whose failure is likely and whose delivery would delay recovery: filters, separator elements, oil, belts, drain valves, sensors and a contactor or two, chosen for the exact model. Identify each part from the nameplate and service records, and review the list after every breakdown so it reflects real failures.
Related pages
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
Air criticality priority score
Score each air-user group by consequence and likelihood of losing air, and flag any severe consequence that hangs on a single point of failure.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Consequence level of each group (1–5): 5, 3, 2, 1
- Likelihood of loss of each group’s supply path (1–5): 3, 3, 3, 3
- Single point of failure in the supply path? (1 = yes, 0 = no, per group): 1, 0, 0, 0
Result
- Group 1 priority score: 15 of 25 (5 × 3) – FLAGGED: severe consequence with a single point of failure
- Group 2 priority score: 9 of 25 (3 × 3)
- Group 3 priority score: 6 of 25 (2 × 3)
- Group 4 priority score: 3 of 25 (1 × 3)
- Priority order: Group 1, Group 2, Group 3, Group 4 highest score first
- Flagged groups: Group 1 record the protection chosen and how it will be tested
Method. Enter one value per user group, in the same order in every list. Priority score = consequence (1–5) × likelihood of loss of the supply path (1–5). Any group with consequence 5 that depends on a single point of failure (enter 1) is flagged whatever its score. The score ranks where standby, storage, spares or rental cover are worth considering; it does not prove reliability. Default: the page example (instrument air, packaging, cleaning, workshop).
What to take from it. Act on the flagged groups first, then the highest scores, and write down the protection chosen and how it will be tested.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.
N+1 capacity and storage ride-through check
Check whether the plant can meet demand with its largest compressor out of service, how much load must be shed if not, and how long the receiver bridges the gap.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Capacity of each compressor at the required pressure (m³/min): 6, 6
- Normal demand (m³/min): 9
- Demand that can be shed during an outage (m³/min): 0
- Receiver volume (m³): 3
- Normal minimum pressure (bar g): 7
- Lowest pressure the critical users accept (bar g): 6
Result
- Total installed capacity: 12 m³/min
- Capacity with the largest unit out: 6 m³/min
- Shortfall with the largest unit out: 3 m³/min
- N+1 on compressors: Not met: 3 m³/min short; shed load or add capacity
- Usable stored free air: 2.96 m³ (3 m³ × 1 bar ÷ 1.013)
- Ride-through if all air is lost: 19.7 seconds at 9 m³/min
- Ride-through with the largest unit out: 59.2 seconds at the 3 m³/min shortfall
Method. Shortfall with the largest unit out = demand − (total capacity − largest unit). Usable stored free air ≈ receiver volume × allowed pressure fall ÷ 1.013 bar (isothermal). Ride-through time = stored free air ÷ the flow not being supplied. Use capacities measured at the required pressure. This checks compressors only: dryers, controls, power and headers need their own N+1 test. Default: the page example (two 6 m³/min machines, 9 m³/min demand, 3 m³ receiver, 7 → 6 bar g).
What to take from it. Two machines are not N+1 unless the remaining ones carry the critical demand. Storage buys seconds for a standby start; it does not replace a missing machine.