Why specialist systems fail at the interfaces
Most specialist systems are assembled from several packages bought from different suppliers: a compressor or vacuum pump, a dryer or purifier, receivers, coolers, a control panel, instruments and pipework. Each supplier designs and guarantees its own scope. The gaps appear between them.
- The compressor maker assumes a cooling-water temperature that the plant cannot deliver in May.
- The dryer supplier assumes an inlet temperature that the aftercooler cannot achieve at a 45 °C ambient.
- The control panel trips the compressor on a signal from a gas analyser, but nobody has decided who sets the trip value or tests the loop.
- The piping contractor routes a vent to a location that the process safety study never reviewed.
- Two suppliers each believe the other is providing the non-return valve between their packages.
A purchase order that lists several suppliers does not establish who owns the complete design. Unless that role is named in writing, it defaults to nobody, and problems surface during commissioning when they are most expensive to fix.
Who should be the system design authority?
The design authority is the party accountable for the integrated system meeting its design basis. It may be the owner’s engineering team, an EPC contractor, a specialist package integrator or a consultant, but it must be competent in the process concerned and have authority to approve or reject interface decisions.
A practical responsibility matrix separates four roles for each deliverable:
| Role | Meaning | Example on a nitrogen booster system |
|---|---|---|
| Accountable | Makes the final decision and signs it off | Owner’s project engineer approves the design basis and trip schedule |
| Responsible | Produces the work | Booster supplier selects the machine and provides performance data |
| Consulted | Gives technical input before the decision | Process safety engineer, instrumentation engineer, operations |
| Informed | Is told the outcome | Maintenance team, insurer, site safety officer |
There must be exactly one accountable party for each item. Two accountable parties usually means none in practice.
Defining the boundaries
Draw every package boundary on a marked-up process and instrumentation diagram. For each boundary state the physical point (a flange, a terminal strip, a valve), the conditions that cross it (pressure, temperature, flow, quality, signals) and who supplies what at that point.
| Interface | What must be agreed | Typical owner of the decision |
|---|---|---|
| Process to package | Design basis: gas, flow, pressure, temperature, quality, operating cases | Process owner or licensor |
| Package to package | Flange ratings, conditions at the joint, isolation and check valves | Design authority, with both suppliers consulted |
| Package to utilities | Power supply, cooling water, instrument air, nitrogen, drains and vents | Design authority and plant utilities team |
| Control and protection | Signal lists, trip values, permissives, alarms, cause-and-effect chart | Controls engineer, with process safety approval |
| Civil and structural | Foundations, loads, vibration, access and lifting | Structural engineer |
| Safety and regulatory | Hazard study actions, hazardous-area classification, pressure-equipment rules | Process safety lead and named competent persons |
Design inputs and the interface register
The design authority keeps two living documents. The first is the design basis: a controlled list of every input with its source and revision. The second is the interface register: one line per interface, with the agreed data, the responsible parties, open questions and closure status.
- Issue the design basis to every supplier at the same revision before they quote.
- Collect each supplier’s datasheet and list the assumptions they made at their boundaries.
- Compare assumptions across suppliers; any mismatch becomes an open interface item.
- Resolve each item in writing, with the accountable party’s approval, and update the register.
- Re-issue affected documents and confirm each supplier has acknowledged the change.
- Do not release equipment for manufacture while interface items that affect it remain open, unless the risk is explicitly accepted.
Hazard review and safeguard ownership
Specialist systems, such as those handling flammable gas, oxygen-enriched streams, breathing air, high-pressure gas or deep vacuum, need a formal hazard study by competent people. The study produces actions and safeguards. The design authority’s job is to make sure each safeguard has a named owner, is designed, installed, tested and recorded.
- Every trip or interlock in the hazard study appears on the cause-and-effect chart.
- Every relief valve has a sizing basis, a set pressure and a safe discharge location.
- Every vent and drain has a reviewed destination.
- Every bypass or override has a procedure, an authorisation level and a record.
- Closure of each action is recorded with evidence, not just a tick.
An equipment supplier, including a compressor service company, cannot by itself certify a medical gas, breathing-air, diving, hazardous-area, cryogenic or oxygen system. Those approvals belong to the qualified parties named for that purpose under the relevant standards and regulations.
Managing deviations and changes
Deviations are inevitable: a substitute valve, a changed cooler, a different motor frame, a site reroute of pipework. Each must be raised, assessed for its effect across all interfaces, and approved or rejected by the accountable party before it is built. Informal site changes are the most common reason why a system that passed design review behaves differently in service.
A simple change record states what is proposed, why, which interfaces and safeguards it touches, who assessed it, who approved it and which documents were updated. Supplier guarantees should not be altered by another party; if a change affects a guaranteed parameter, the supplier must accept it in writing.
Temporary arrangements need the same discipline. A rental compressor or generator brought in for commissioning, a temporary hose replacing a pipe spool or a bypass fitted to keep production running all change the system. Record who approved each one, the conditions for its use and the date by which it will be removed, and check that it is removed before final acceptance.
Integrated testing and handover
Factory tests prove each package. Only an integrated site test proves the system. The handover sequence used on well-run projects looks like this:
- Mechanical completion: installation checked against drawings, pressure tests and cleanliness checks done, punch list recorded.
- Pre-commissioning: loop checks, motor rotation, utility availability, instrument calibration certificates.
- Function tests: every trip, alarm and permissive tested from the field device to the final action.
- Commissioning on a safe medium: air or nitrogen where the process allows, before introducing the process gas.
- Performance test: agreed duty points, measured to an agreed method, compared with guarantees.
- Handover dossier: as-built drawings, datasheets, test records, hazard-study closure, operating and maintenance manuals, spares list and the record of approved exceptions.
- Training and acceptance: operators and maintenance staff trained; owner signs acceptance with any open items and their deadlines listed.
How Reddy Compressors helps
Reddy Compressors has worked in industrial compressed air since 2002 from our office at Tarnaka, Secunderabad. We sell new and used/refurbished compressors, blowers and gas compressors, rent them, service them and supply spares as an independent multi-brand specialist, not an authorised dealer. We have tie-ups with all major and reputed brands. Our team of more than 20 service engineers and technicians covers all of Telangana and Andhra Pradesh, and we supply across India.
On specialist projects our role is clearly defined as a package supplier, rental provider or service contractor within the system, working to the design authority’s basis. We help by:
- Providing compressor, blower or vacuum package data and assumptions clearly at each boundary.
- Flagging mismatches we see between the design basis and site conditions, such as high summer ambient, cooling water and dust.
- Installing, commissioning and servicing our scope, and supporting integrated tests.
- Supplying temporary compressors and, through Reddy Generators under the same management, generator sets for commissioning and temporary utility needs.
Our projects include a temporary power-and-air package for a major natural-gas development at Kakinada, where our team supplied compressors and generators with distribution boards, operators, a technician, erection, commissioning and transport. Specialist approvals for breathing air, medical gas, oxygen, diving or hazardous-area systems remain with the appointed qualified parties.
What to send us
- The design basis or process duty for the compressor, blower or vacuum scope.
- The marked-up process diagram showing our boundary.
- Utilities available: power supply, cooling, instrument air.
- Site location, ambient range and any hazardous-area classification.
- Programme dates for installation, commissioning and handover.
- Name of the design authority and the person who approves interface decisions.
Frequently asked questions
Who is responsible for the overall design of a multi-supplier compressor system?
The party appointed as system design authority, which should be named in the contract. It may be the owner, an EPC contractor or a specialist integrator. Each supplier remains responsible for its own package, but someone must own how the packages work together, the utilities, the controls and the safety functions. If no one is named, interface problems tend to surface during commissioning.
What is an interface register on a project?
An interface register is a controlled list of every connection point between packages, utilities, controls and structures. Each line records the agreed conditions, the parties involved, open questions and closure status. It lets the design authority see at a glance which interfaces are agreed and which still threaten the programme, and it gives suppliers a single reference instead of scattered emails.
What documents should a compressor package handover include?
A complete handover normally includes as-built drawings, datasheets, factory and site test records, instrument calibration certificates, trip and alarm test sheets, hazard-study closure records, operating and maintenance manuals, a recommended spares list and a record of approved deviations and open items. Without these, the owner cannot maintain, modify or troubleshoot the system confidently.
Can a compressor supplier certify a breathing-air or medical-gas system?
Not on its own authority. A compressor supplier can provide equipment and data, but certification of breathing-air, medical-gas, diving or oxygen systems belongs to competent parties working to the applicable standards and regulations. The design authority should appoint them early so that their requirements shape the design rather than being checked only at the end.
Why test the whole system if each package passed its factory test?
Factory tests run each package under the maker’s conditions, often with its own utilities and controls. On site the packages meet real cooling water, real power quality, real ambient heat and each other’s controls. Integrated testing proves that trips, permissives and duty points work together as designed, which no individual factory test can show.
How should site changes be handled during installation?
Every change should be raised in writing, checked for its effect on all connected interfaces and safeguards, approved by the accountable party and recorded before it is built. Affected drawings and datasheets are then updated. If the change touches a guaranteed parameter, the supplier concerned must accept it in writing so that the guarantee remains clear.
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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.