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ASME Standard 120-1969 — Recommended Practices for the Design of Gas Turbine Generator Lubricating Oil Systems
⚙️ ASME Standard No. 120 (1969), formally titled Recommended Practices for the Design of Gas Turbine Generator Lubricating Oil Systems, was prepared by Panel IV of the ASTM-ASME-NEMA Joint Committee on Turbine Lubricating Systems. It was presented and reviewed at the June 1967 ASTM annual meeting in Boston, and received sponsor approval by ASTM on November 3, 1967, by NEMA on January 31, 1968, and by ASME on March 18, 1969. Its purpose is to aid the equipment builder, the erector, the oil supplier, and the operator in coordinating their efforts toward satisfactory oil-system performance — representing the generally accepted minimum design basis for gas- and steam-turbine power-plant lubrication engineering at the close of the 1960s.
💡 Unlike the marine series (111 and 115), Standard No. 120 targets land-based and stationary gas-turbine generator units, and it elevates the control-oil and seal-oil functions to the same importance as lubrication — the defining characteristic of a gas-turbine supply system.
1. Scope and Objective 📌
Section 1.0 states the scope: the recommended practices apply to the design of lubricating oil systems for gas-turbine-driven generator units of 1000 kW and larger. The lubricating oil system is defined as the complex that utilizes and circulates the turbine-generator lubricating oil and furnishes pressurized oil for control and seal functions.
The objectives are to establish minimum recommended design practices so that:
- lubrication, control, and seal functions are performed satisfactorily by the oil mutually acceptable to the parties concerned;
- installation, cleaning, and flushing are facilitated;
- satisfactory system cleanliness can be maintained;
- safe practices are observed.
⚠️ The applicability is bounded at 1000 kW and does not automatically extend to all unit sizes. Smaller units or unusual arrangements should be evaluated against the manufacturer’s specifications.
2. System Overview 📊
The operation of the gas-turbine generator depends upon a satisfactory supply of lubricating oil at the proper places, so a highly reliable system must be supplied. The system flow requirements are the summation of the individual requirements for lubrication of all bearings, gear meshes, and couplings supplied by the system, plus the steady-state and transient control-oil requirements, with an added margin for flow changes with use.
The system pressure must be sufficient to overcome piping and equipment pressure drop, overcome elevation head difference, provide margin for regulation, and insure proper distribution of lube oil to the required areas of the machinery. The control and seal-oil function may require higher pressure levels than lubrication; for these cases the total system must be designed accordingly with boosting and pressure-stabilizing provisions.
| Design Item | Key Requirement | Engineering Implication |
|---|---|---|
| System flow | Bearings + gears + couplings + control oil + margin | Insufficient flow overheats bearings and gears |
| System pressure | Overcome drop and head, leave regulation margin | Low pressure impairs distribution |
| Control/seal oil pressure | May exceed lube pressure | Separate boost and stabilization needed |
| Reliability | High-reliability system design | Standby pump, accumulators, transfer logic |
3. Materials, Reservoirs, and Pumps 🔩
The materials guidance is consistent with the preceding series: minimize copper, zinc, lead, and similar metals that catalyze oil oxidation; prefer steel for piping, valves, and fittings with stainless steel acceptable; use high tin-base babbitt for bearing linings; prefer steel plate for tanks; and select oil-resistant gaskets, seals, and hoses. The Oil Reservoirs chapter sets out tank volume, deaeration of return oil, baffling, drain, and access provisions; the Pumps chapter requires the configuration and capacity of main and standby pumps to maintain supply continuity during idle and shutdown phases of the unit.
4. Coolers, Filters, and Regulation 🌡️
The Coolers chapter ensures the oil outlet temperature stays within the manufacturer’s limits, with twin coolers where appropriate for on-line changeover. The Filters chapter protects the cleanliness of the oil system with easily serviced elements. Regulation maintains stable supply pressure and flow — including pressure-regulating valves, thermostatic control, and necessary bypass loops. The Instrumentation chapter requires monitoring and alarming of oil pressure, oil temperature, oil level, and filter differential pressure, giving operators complete condition information.
🚨 On a gas turbine, the lubricating oil also serves control and sealing duties: a loss of supply damages not only the bearings but can directly cause uncontrolled shutdown or equipment damage. Redundancy of pumps, coolers, and filters and the transfer logic must be verified.
5. Piping, Cleaning, and Flushing 🛠️
The Piping chapter requires welded or flanged connections properly braced and routed away from hot surfaces. The Standard places particular emphasis on post-installation cleaning, flushing, and purification, because gas-turbine bearings and control valves are extremely sensitive to particulate contamination in the oil. Acceptance after flushing should be based on filter/strainer inspection and measured oil cleanliness.
6. Engineering Practice Essentials ✅
- 🔍 Demand list: calculate system flow and pressure item by item — bearing lubrication, gears/couplings, control oil, seal oil, plus margin.
- 🔧 Redundancy: main plus standby pump, dual coolers, and filter changeover to guarantee uninterrupted supply over the service life.
- 🌡️ Temperature control: combine cooler capacity with thermostatic regulation to hold the manufacturer’s outlet-temperature window.
- 🧹 Cleanliness acceptance: complete system flushing before commissioning; accept on strainer inspection and oil-sample cleanliness.
- 📋 Condition monitoring: fully instrument oil pressure, temperature, level, and filter differential pressure with alarms.
7. Frequently Asked Questions ❓
❓ Q: What unit size does Standard No. 120 cover?
A: Section 1.0 states it applies to gas-turbine-driven generator units of 1000 kW and larger; smaller units may follow its design philosophy but should be assessed against the manufacturer’s specifications.
A: Section 1.0 states it applies to gas-turbine-driven generator units of 1000 kW and larger; smaller units may follow its design philosophy but should be assessed against the manufacturer’s specifications.
❓ Q: Why single out control oil and seal oil?
A: Unlike pure lubrication, a gas turbine’s fuel/gas control actuators and seals are driven by pressurized oil, and their pressure requirements often exceed those of lubrication — so the system must be designed and guaranteed for the higher pressure levels.
A: Unlike pure lubrication, a gas turbine’s fuel/gas control actuators and seals are driven by pressurized oil, and their pressure requirements often exceed those of lubrication — so the system must be designed and guaranteed for the higher pressure levels.
❓ Q: Why is system cleanliness so important?
A: Gas-turbine bearing clearances are small and control valves precise; particles in the oil quickly cause wear and sticking. The Standard therefore makes post-installation flushing and cleanliness maintenance explicit objectives, accepted by strainer inspection and oil sampling.
A: Gas-turbine bearing clearances are small and control valves precise; particles in the oil quickly cause wear and sticking. The Standard therefore makes post-installation flushing and cleanliness maintenance explicit objectives, accepted by strainer inspection and oil sampling.
❓ Q: Does this 1969 Standard still have value today?
A: Its overall architecture — system definition, flow and pressure accounting, redundant standby, temperature and cleanliness control — remains the common foundation of modern gas-turbine and combined-cycle oil-system design, with the specifics developed and refined by current manufacturer and engineering standards.
A: Its overall architecture — system definition, flow and pressure accounting, redundant standby, temperature and cleanliness control — remains the common foundation of modern gas-turbine and combined-cycle oil-system design, with the specifics developed and refined by current manufacturer and engineering standards.
