A HAZOP requiring two dissimilar non-slam check valves downstream of a reciprocating compressor calls for two independently justified line-valve designs, not merely two valves with different labels. A recycle line can make the compressor’s internal inlet and discharge valves unsuitable as the credited backflow barrier. Check the required safety function and operating envelope before selecting a check-nozzle valve or a swing check with a slam retarder.
Discharge backflow symptoms and their causes
Backflow can show up as continued compressor operation when it should stop, reverse flow through the discharge line, or a check valve that fails to isolate. These observations do not identify a single cause; relate each symptom to the equipment boundary and the operating mode.
| Observed condition | Possible mechanism | Decision to make |
|---|---|---|
| The compressor continues running after the process no longer needs delivery. | Leakage through a discharge path can permit flow back toward the compressor. One reported installation needed a discharge check because leakage through integrated leaf-type valves allowed some backflow. | Measure or otherwise establish the direction and amount of flow in the operating state that triggers the concern. Confirm whether the line check must prevent reverse flow, provide gas-tight isolation, or both. |
| Internal compressor valves are proposed as the only backflow barrier. | Internal inlet and outlet valves cycle with compressor strokes and can have different failure visibility than a piping check. A recycle path may also defeat the assumed boundary. | Map the recycle connection and identify whether the HAZOP action applies to the external discharge line or the compressor stages. |
| A line check is open during forward flow and its failure is not apparent. | A check valve normally remains open while flow is forward; a stuck-open failure can therefore be hidden until reverse flow is demanded. | Assess how the installation detects failure on demand and whether the check is credited as an independent protection layer. |
| Valve closure produces impact or noise as flow reverses. | A moving closure element may not seat before reverse velocity develops, causing slam. | Review dynamic closure performance for the compressor’s pulsating flow, not just nominal flow and pressure ratings. |
The discussion of whether a reciprocating compressor’s internal valves make a discharge check unnecessary reflects different system boundaries and duties. Some compressors use internal valves that cycle each stroke, and individual failure may stop operation or become evident. That does not establish that the valves provide the required external isolation in a system with a recycle line. Resolve the boundary and credited safety function with the process hazard review.
Reciprocating flow and non-slam closure
Reciprocating compressors create pulsating discharge flow rather than a perfectly steady stream. A check valve responds to local flow and pressure: forward differential pressure opens it, and the closing mechanism must return the closure element to its seat as forward flow decays. If the element is still moving when reverse flow has accelerated, the reverse stream can drive it into the seat and cause slam.
“Non-slam” describes a closure behavior intended to limit that impact; it is not a universal performance guarantee. A valve that behaves acceptably at one flow rate or pulse pattern may not do so over the full operating range. The valve supplier’s sizing and dynamic application review should use actual gas conditions and compressor operating cases.
Also separate two requirements that can be confused: preventing reverse flow and providing tight shutoff. A valve selected to avoid slam is not automatically gas-tight. Specify the allowable leakage or isolation duty separately, then check whether the proposed design meets it.
Meaning of two dissimilar protection devices
For a HAZOP action, “dissimilar” should mean more than different manufacturer names. Different valve architectures can reduce shared design vulnerabilities, but different architectures do not by themselves prove independence. The hazard review should define what diversity it requires and identify credible common-cause failures, such as shared operating conditions, installation errors, maintenance practices, or a common blockage.
Record the credited function for each valve: reverse-flow prevention, tight isolation, non-slam closure, or a defined combination. Then confirm how the arrangement is tested and how a hidden stuck-open failure would be detected. If the action requires two independent barriers, ask the process safety authority to approve the independence basis rather than treating a second valve as automatically doubling protection.
An alternative sometimes considered is detecting loss of forward flow with process transmitters and closing a fast-acting isolation valve. One suggested architecture uses two-out-of-three voting. That is an instrumented protection concept, not a drop-in check-valve substitute: it requires a defined trip basis, testing and calibration, and coordination with automatic depressurization, relief devices, and other protection systems. Use it only through the applicable hazard review and design process.
Application data for valve selection
Before asking manufacturers to recommend a design, assemble a common application basis. Without it, proposals cannot be compared on the same duty, and “non-slam” can become a label instead of a verified performance requirement.
- Define the protected boundary. Show the compressor discharge, recycle connection, other flow paths, and the location of both proposed check valves. State the HAZOP scenario and the required response to reverse flow.
- Document operating cases. Provide gas composition and properties, pressure and temperature range, flow range, compressor operating states, and relevant pulsation information. Obtain the values from process data and compressor documentation rather than estimating them from a nominal rating.
- Separate performance requirements. State the allowable pressure drop, required shutoff or leakage performance, acceptable closure behavior, installation constraints, and any inspection or test requirements. Set project acceptance criteria before comparing bids.
- Request a dynamic review. Ask each supplier to assess the valve against the compressor’s actual operating cases, including minimum and changing forward flow and the expected reverse-flow demand. Obtain the assumptions and limitations in writing.
- Document diversity. Compare the two proposed designs, their closure mechanisms, likely failure modes, and maintenance dependencies. Submit the resulting independence rationale to the authority responsible for the HAZOP action.
Candidate check-valve architectures
Two candidate approaches mentioned for this application are a check-nozzle type valve and a swing check fitted with a slam retarder. They are alternatives to evaluate, not interchangeable selections. Ask suppliers to demonstrate suitability for the actual pulsating gas service and explain how each design achieves the requested closure behavior.
| Candidate | Selection questions | Evidence to request |
|---|---|---|
| Check-nozzle type | Does the proposed size and design cover the operating envelope, including low or changing flow? Does its construction meet the separate leakage duty? | Application-specific sizing and dynamic-performance basis, pressure-drop information, and shutoff or leakage data for the specified duty. |
| Swing check with slam retarder | How does the retarder affect closure over the compressor’s flow cycle? Is the installation orientation compatible with the proposed design? | Supplier explanation of closure performance for the stated pulsation and installation, along with pressure-drop and leakage information. |
A known supplier or prior successful use is a starting point for a technical inquiry, not proof of fit. Compressor-valve manufacturers may offer different internal valve constructions, but an internal compressor valve and an external line check have different locations and functions. Specify the required external valve architecture and duty directly.
Numbered commissioning and verification checks
Commission against written acceptance criteria. A successful open/close movement alone does not demonstrate non-slam performance, gas-tight isolation, or the independence required by the HAZOP.
- Confirm identity and installation. Compare installed valve tags and construction records with the approved dissimilarity basis. Expected reading: each installed valve matches its approved architecture and the documented flow direction and orientation.
- Check operating differential and flow. Record the relevant process conditions at representative operating cases. Expected reading: each valve opens and remains stable over the specified forward-flow range without exceeding the project’s pressure-drop limit.
- Observe closure during a defined operating transition. Use the approved commissioning method and suitable instrumentation or observation. Expected reading: closure occurs without unacceptable impact or reverse-flow behavior under the tested case. Record the case and measured result; do not infer full-range performance from one test point.
- Test the isolation duty. Apply the specified leakage or seat test procedure. Expected reading: measured leakage meets the project acceptance value at the stated test conditions. If no value is specified, resolve that requirement before crediting the valve for tight isolation.
- Verify failure detection and testability. Confirm the inspection, proof-test, or monitoring method for a hidden stuck-open condition. Expected reading: the documented method can identify failure of the credited function, and the test record names the acceptance criterion and result.
Recurring selection and protection pitfalls
- Crediting two checks without a failure analysis: a second check may add little protection if both share a hidden failure mode or if neither failure is detected before demand.
- Assuming internal valves settle the recycle-line question: map actual flow paths and the HAZOP boundary. Internal stage valves may not isolate the external discharge line under the required scenario.
- Treating “non-slam” as a valve size or guarantee: closure depends on operating dynamics. Get an application-specific review and define what performance will be accepted.
- Equating non-slam with gas-tight: specify closure impact and leakage limits as separate criteria.
- Substituting a control scheme without revising the safety basis: a transmitter-and-shutoff-valve scheme must preserve proper operation of depressurization, relief, and other instrumented protections and must be tested and monitored.
FAQ
What happens if I install two standard check valves in series?
They may provide two physical devices without providing two independent protection layers. Check for hidden stuck-open failure, common-cause vulnerabilities, and a defined way to test the credited reverse-flow function.
What happens if the compressor has internal discharge check valves?
Those valves may cycle each stroke and make some failures evident, but they do not automatically meet an external discharge isolation requirement. A recycle line can change the flow path, so review the HAZOP boundary and actual piping arrangement.
How do I verify that a non-slam check valve is suitable?
Compare the installed valve with the approved design, record performance at representative operating cases, and check closure behavior against written acceptance criteria. Finish by testing leakage to the specified limit and recording the result and test conditions.