Lube Oil System

In rotary equipment, lubrication is one the most important factors that shall be considered. Lubrication will reduce the friction during the operation. Reduced friction will results in higher efficiency, longer maintenance intervolves and lower maintenance costs. There are multiple standards for lube oil units but the most famous one is API 614. This standard includes 4 chapters; the first chapter is general requirements that shall be applied to all other sections. Second chapter are for special purpose and the third chapter is for general purpose lube oil system, both of these terms are defined in the next chapter. The fourth chapter specifies the dry gas seals requirements. There are multiple methods for lubrication. In this article we will discuss “Forced Lubrication”. A forced lubrication system consists of few elements such as Pumps, strainer, filters, coolers, rundown tank, and reservoir.

Special purpose Vs. General purpose

As stated in API 614, “The term "special-purpose application" is defined in ISO 10438-1.”. However, other standards have their own definitions as well. For example for turbines, as stated in API611 “General-purpose turbines are horizontal or vertical turbines used to drive equipment that is usually spared, is relatively small in size (power), or is in non-critical service. They are generally used where steam conditions will not exceed a pressure of 48 bar (700 psig) and a temperature of 400 °C (750 °F) or where speed will not exceed 6,000 r/min.”.

In our company, I think it is applicable to other companies as well, any equipment that has following conditions is considered as special purpose:

1. The operation of the equipment is critical; meaning that if the equipment fails whole plant will shut down or it cause a hazard to environment or personnel.
2. The equipment does not have a stand by. For example in methanol plants, Synthesis gas compressor, also called SYNGAS compressor, is a critical unit that if fails, whole plant will shut down, and due to high initial costs, no stand by compressor is considered. Thus SYNGAS compressor is considered as special purpose equipment and all auxiliaries shall be purchased considering this criterion, for example turbines, coupling, gearboxes and lube oils shall be all special type. On the other hand boiler feed water pumps are critical as well but they might have standby unit. In this case they can be considered as general purpose units.

Special purpose units have more regulations and restrictions. They are more expensive but the break down probability is lower than general purpose units.

For the rest of article, we follow the special purpose standard since it has more restriction than general purpose lube oil units.

Lube Oil System elements

1- Base plate

It is a plate that every other equipment mounts on it. Base plate can be one plate that all elements mount on it or multiple plate based on available location.

2- Reservoir

The storage location for oil. The reservoir shall have multiple facilities including:

• Level gauge: to show oil level in tank
• Heater: to heat the oil in the cold weather.
• Man hole: For maintenance reasons,
• Connection: Forced lube oil system is a closed system. Oil shall return to reservoir after it done lubrication. All the return lines shall be equipped with a flow indicator device. Due to lower costs, manufacturers are usually uses sight glass. Operators shall be able to see the returning of oil.
•  Spare Connection: This is our company practice as well. Who knows what will happen in future? It is easier to have a spare connection during the reservoir manufacturing just in case it is needed in future.
• Oil purifier/Oil conditioner connection: Based on manufacturer practice this connection can be same as spare connection. During the operation the quality of oil will drop, especially in turbines, there is a risk that steam enter the oil. Water droplets in oil are not acceptable. In figure 1 a typical oil purifier is shown. Its duty is to separate water from oil. The purifier works based on centrifugal principals. Purifier rotates oil at high speed. Due to centrifugal forces oil and water will divide into two phases. Then the water will routed to drain. Again in our company it is normal practice to consider one “PORTABLE OIL PURIFIER” or maximum two for whole plant but some manufacturer such as SIEMENS, will use one fixed purifier. If the purifier is fixed, then there is no need for any additional connections.

Figure 1- Portable Centrifugal oil purifier

• Drain connection: As the name suggests, this connection is used for draining the oil into drainage channel. It is important that the bottom of the reservoir have slope to the drain so solid particles can be drained easily.
• Filling nozzle: This nozzle is used to fill the reservoir.

A typical P&ID (Piping and Instrumentation Diagram) of lube oil reservoir with its auxiliaries are shown in figure 2.

Figure 2- Typical reservoir P&ID

3- Oil pumps

Depending on oil viscosity, required pressure, required flow and other working conditions manufacturer use different type of pumps. Screw pumps are normal selection while other pumps such as gear pump or centrifugal are other selections. At the suction of the pumps, manufactures are always provide strainers. These strainers will prevent large solid particles to enter the pump and damage it. These strainers shall not be confused with filters. Strainers will only filter solid particles with large diameter.

 Selection of the pumps are important but there is one more important factor that shall be considered; POWER!. Source of pumps power are utmost important. In special purpose lube oil skid, we consider 3 pumps; Primary pump, secondary pump and emergency pump. Primary pump, as the name implies, is the pump that always works. Secondary pump comes into service when the supplied pressure from the first pump is not sufficient. When the secondary pump comes to service and if the supplied pressure is not sufficient, two actions set at motion together:

1. Device will receive trip signal.
2. Emergency pump will come to service.

It is important that these three pumps have different source of power. If steam is available then the suggested combination is as follow:

• Primary pump on steam (using general purpose turbine)
• Secondary pump on AC motor from Electrical grid. Standby pump shall be always motor-driven due to fast start up. Turbine driven pumps requires warming which is not acceptable for secondary pumps.
• Emergency pump on AC from Diesel generator. Emergency pump have a different oiling route. This pump is not used for normal lubrication but rather in emergency case. It will provide oil with sufficient flow and pressure during SHUTDOWN to avoid bearing damages and to provide uniform cool down. A route of emergency oil pump is show on Figure 3.

Figure 3- Main and secondary oil pumps route (red) Vs. Emergency oil pump route (green)

There are other sources of power to be considered as well for pumps

• UPS: Batteries: Simple but they require regular maintenance and may not be reliable as other sources
• Shaft driven pumps: These pumps are driven using the main equipment shaft. The major problem for these pumps is maintenance. In order to do repair on these pumps, whole equipment shall be shut down. On the other hand since these pumps work as long as shaft rotates, in the case of shutdown these pumps provide oil to bearing until the shaft stops completely.

For shaft driven pumps, maintenance, NPSH, priming and thermal growth shall be considered.

4- Coolers

A twin parallel coolers shall be provided with transfer valve. Only one of coolers is in service while the other one is on standby. These coolers shall be connected with a line and orifice. Standby cooler shall be always prime for immediate startup. A line with orifice connects these two cooler. Due to high pressure in working cooler, standby cooler will be filled with oil. There are lines that connect coolers to reservoir. After the standby cooler is filled with oil, excess oil in the standby cooler returns to reservoir. This will keep standby cooler always prime. Shell and tube coolers are preferred but other type of coolers can be accepted upon agreement between purchase and manufacturer.

According to our experience, we consider a TSV (Temperature safety valve) and a sight glass on coolers water outlet. If the temperature rises within the cooler, TSV will protect the cooler and if tubes are blocked, operator can see through the sight glass, that there are no water flow.

Figure 4- Twin lube oil system coolers

5- Filters

Twin filters are provided to prevent solid particles reach the critical parts of the equipment. One filter is in the service while the other filter is standby. Priming of the filters is same as coolers, described in previous chapter.

For ease of maintenance, filters shall have replaceable filtering component. As stated in API 614 “Filters shall provide a minimum particle removal efficiency, EPR of 90 % for 10 μm particles (β10≥10) and a minimum EPR of 99,5 % for 15 μm particles (β15≥200), both in accordance with ISO 4572 when tested to a minimum terminal (end-of test- run) differential pressure of 350 kPa (3,5 bar; 50 psi).”

Filter shall be downstream of coolers in order to filter if any solid part from coolers enters the oil lines.

Figure 5- Oil filters. Connecting line between filters are clearly shown in this picture (middle line)

6- Rundown tanks

Rundown tank is a smaller reservoir that installed at high locations. If all the pumps fails, or if the diesel generator takes too long to come into service, rundown tank will provide oil with sufficient pressure. The pressure is produce due to gravity. Rundown tanks are equipped with a level gauge, return line that returns excess oil back to reservoir and feed line.

When the pumps are working normally, the rundown tank will be filled. If the inlet pressure drops, then the direction of flow will be reversed and rundown tank empties its oil back to the line. As mentioned earlier considering the emergency pump to be on diesel generator electrical grid; Diesel generators will starts in 30 secs (more or less). Rundown tank shall provide oil for this crucial 30 secs or bearings of the equipment will be damaged. After that the emergency pump starts and provides sufficient lubrication until the equipment cools down completely. If there are no emergency pumps, then the volume of rundown tank shall be sufficient for whole cooldown period, which is around 5-10 minutes.

7- Accumulators

Accumulators provide higher pressure than rundown tank but due to cost, they will be used only in small volumes. They are mainly used on control lines where 1- The pressure is higher than lubrication lines 2- required oil volume is lower.

Figure 6- Accumulator

 Alarms and Shut downs

In the API, there are couple of alarms and shut downs defined for different scenarios. These scenarios are shown in below table:

Figure 7- Alarms and shut down according to API 614 Chapter 2

Common faults in lube oil units

1. Contamination: It happens when steam enters the oil lines and water returns to reservoir. If the amount of water is small, it can be separated with purifier. If the water content is too much, then the root of problem shall be identified
2. Lube oil elements break down: It is possible, and normal, that mechanical equipment have break down. This is the reason everything has a standby. Standby element will come into service while the primary element will be sent for repair. Coolers, Filters, Pumps all have standby unit.

Related Articles

1. Centrifugal Compressor- Part IV- Dry Gas Seals
2. Centrifugal Pumps- Part II- NPSH and Cavitation

References

1. API 614- 5th edition
2. Cover picture KGD group
3. Figure 1- Hydraulic Oil Filter
4. Figure 4- PSI Engineering
5. Figure 5- 3Lfilter
6. Figure 6- Jabsco Accumulators

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