The rated point sits outside the plotted performance curve and close to the high-discharge-pressure end. That is not an acceptable basis for selection: the curve must contain the required flow and discharge-pressure point under the stated operating conditions, or the manufacturer must issue a guaranteed performance point backed by a corrected curve.
Pressure, flow, and thermal load
The number that matters is the differential pressure across the compressor at the required gas flow. As discharge pressure rises, the machine must transfer more energy to the gas and seal liquid. Shaft power, discharge temperature, seal-liquid temperature, and internal hydraulic loading can rise while achievable flow falls.
This is heat, not logic. A plotted endpoint normally marks a tested, calculated, or permitted operating boundary. A required point beyond that boundary calls for extrapolation, and extrapolation near a curve limit can hide a steep increase in power or loss of stable capacity.
The supplied plot uses discharge pressure on the X-axis and flowrate on the Y-axis, with the rated point identified as RP. Its meaning still depends on inlet pressure, gas composition, seal-liquid temperature, speed, and the definitions of pressure and flow. Without those conditions, geometric proximity to the curve does not establish performance.
Selection approaches and acceptance criteria
| Approach | What it establishes | Main risk | Decision |
|---|---|---|---|
| Accept the point because it is close to the curve | Only visual proximity | Required flow, power, temperature, or pressure capability may be missed | Reject as an engineering acceptance method |
| Extrapolate the existing curve | An estimated point outside the plotted range | Curve shape can change near a hydraulic, thermal, motor, or discharge limit | Use only if the manufacturer formally owns the extrapolation and guarantee |
| Obtain a corrected curve and written guarantee | Performance at the specified duty and stated reference conditions | Residual risk depends on the completeness of the stated conditions | Recommended |
| Select another size or operating arrangement | Moves the duty inside a documented operating envelope | May change power, utilities, footprint, and control requirements | Use when the original selection cannot carry the duty with margin |
Compare NASH, Graham, SIHI, or any other supplier on the same guaranteed duty sheet rather than on reputation alone. The comparison must use identical inlet conditions, discharge conditions, gas basis, seal-liquid conditions, speed basis, auxiliaries, and guarantee tolerances.
Quantities that define the curve
| Quantity or limit | Why it changes the decision | Where to read it |
|---|---|---|
| Inlet absolute pressure | Defines suction density and compression ratio | Process datasheet and curve header |
| Discharge absolute pressure | Sets the opposing pressure and required compression work | Downstream design case and curve axis definition |
| Flow basis | Actual, standard, wet, and dry flow values are not interchangeable | Curve legend and performance guarantee |
| Gas composition and molecular weight | Affect density, power, heat transfer, and condensation behavior | Process composition sheet |
| Seal-liquid type and inlet temperature | Vapor pressure and temperature influence capacity and heat rejection | Utility datasheet and curve correction notes |
| Speed and impeller configuration | Identify the physical configuration represented by the curve | Certified curve and equipment datasheet |
| Absorbed power and driver limit | Show whether the motor and coupling can carry the duty | Power curve, motor nameplate, and supplier data |
| Permitted operating boundary | Separates continuous duty from restricted or prohibited operation | Manufacturer operating map and manual |
Also determine whether the discharge pressure axis is gauge or absolute pressure. Compression ratio calculations require absolute pressures. Read the rated-point power directly from the corrected power curve; a flow-versus-pressure curve alone cannot qualify the driver.
Bearing lubrication and shaft-sleeve decisions
Grease lubrication applies to the bearing system, not to the liquid ring or mechanical-seal faces. A grease-lubricated design can be suitable when bearing load, speed, grease specification, relubrication interval, housing temperature, contamination control, and purge provisions match the service. Too much grease can churn and heat the bearing; too little or contaminated grease shortens bearing life.
Mounting mechanical seals directly on the shaft is also a design choice, not an automatic defect. The acceptance issue is whether the shaft surface provides the hardness, finish, dimensional tolerance, corrosion resistance, and fretting resistance required by the seal arrangement. A sleeve provides a replaceable wearing surface and can simplify restoration after corrosion, scoring, or repeated seal work. Without one, damage at the seal location may require shaft repair or replacement.
Request a sectional drawing showing which seal components contact or clamp to the shaft. Confirm the shaft material, surface treatment, finish, allowable repair method, seal setting procedure, and replacement strategy. Review bearing lubrication and seal construction separately; neither answers whether the hydraulic rated point is valid.
Rated-point qualification procedure
- Mark the required inlet pressure, discharge pressure, and flow on the same basis used by the curve. Convert gauge pressure to absolute pressure before calculating compression ratio.
- Read every condition printed on the curve: gas basis, seal-liquid type and temperature, speed, and any correction notes. Resolve mismatches through a revised manufacturer calculation.
- Request the complete performance set, including capacity, absorbed power, thermal data, and permitted operating boundary. Ask the manufacturer to place
RPon that certified set. - Require written identification of the limiting condition near the right-hand endpoint. It may be a hydraulic, thermal, power, mechanical, or application boundary; the curve image alone does not identify which one.
- Check the maximum and minimum process cases, not only the rated case. Include realistic downstream pressure loss and utility temperature variation from the project data.
- Verify that the driver, coupling, shaft, bearings, seal system, casing, and auxiliaries are rated for every accepted case. Do not operate beyond a stated pressure or mechanical limit because equipment damage or loss of containment can result.
- If the required point remains outside the documented envelope, resize the compressor or change the arrangement. Record the selected point and guarantee conditions in the purchase specification.
Commissioning and performance verification
Establish a baseline after temperatures and seal-liquid conditions stabilize. Record inlet absolute pressure, discharge absolute pressure, gas flow on the contractual basis, speed, motor load, seal-liquid inlet and outlet temperatures, bearing temperatures, vibration, leakage, and valve positions. Compare simultaneous readings with the guaranteed curve rather than comparing measurements taken at different times.
A failed flow guarantee with acceptable discharge pressure can arise from an incorrect flow basis, warmer seal liquid, gas-condition changes, speed error, internal clearance, leakage, or instrument error. Excess motor load points toward a pressure, flow, liquid-rate, mechanical-drag, or data-basis problem. Trending temperatures and motor load while discharge pressure changes reveals whether operation is approaching a thermal or power boundary.
Frequently asked questions
What happens if the rated point is just outside the liquid ring compressor curve?
The duty is unqualified even if the graphical gap looks small. Obtain a corrected curve and written guarantee, or move the duty inside the documented operating envelope.
What happens if discharge pressure reaches the end of the curve?
Flow may fall while absorbed power, seal-liquid temperature, and internal loading rise. Read the endpoint definition and limiting condition from the manufacturer’s operating map before accepting continuous operation.
What happens if the seal liquid runs warmer than the curve condition?
Its vapor pressure rises, which can reduce effective suction capacity and alter thermal performance. Apply the manufacturer’s correction using the measured inlet temperature and specified liquid.
What happens if mechanical seals run directly on an unsleeved shaft?
The shaft becomes part of the seal installation and repair strategy. Verify its material, hardness, finish, corrosion resistance, seal-setting method, and recoverable wear allowance before approval.
When should I stop testing and contact official support?
Stop when the required point lies outside the permitted map, motor load or temperature approaches a stated limit, vibration rises abnormally, or leakage indicates seal distress. Preserve the simultaneous operating data and contact the manufacturer’s official technical support channel for a corrected curve or operating ruling. Resume only after the limiting condition and acceptable duty are documented.