Larry Denk, P.E. and Michael Karlin, Aggreko Process Services; and Daniel Earnest and Loren Bartlett, Allegheny Energy
Coolers are utilized on lubricating oil reservoirs of large rotating equipment to control viscosity and reject heat. Lube oil cooler performance is impacted by a number of plant-specific variables, such as ambient temperature, plant utilities, equipment condition, and the like. This article presents design considerations, an engineered approach and plant case project for cost-effective, temporary, supplemental heat rejection from large lube oil reservoirs.
The success of the approach depends, in large part, on an accurate engineering understanding of heat transfer in lubricating oil systems. Performance predictions from commercially available computer models had been found to be misleading. A spreadsheet-based model that included mass/heat transport in viscous, laminar flow was found to be quite accurate. Field data taken during operation confirmed the conclusions and validated the spreadsheet model.
In a full scale application, Allegheny Energy collaborated with Aggreko to provide temporary, supplemental lube oil cooling for the No. 1 and No. 2 turbines at the Pleasants, W.Va., power station. Each of two temporary systems was designed to reject an additional increment of about 2 MMBtuH from the lube oil reservoirs to the plant cooling water system. The projects were installed and on-line within 11 days following the decision to proceed.
The success of the projects confirms that temporary, supplemental cooling can be rapidly and safely implemented. This also permits conservation of capital and avoids permanent equipment that would only be used during a small portion of the year.
Viscosity (and thus lubricity) control of lubricating oil in large rotating equipment service is important for bearing life, controlled maintenance and operating costs and machine reliability. The generally accepted practice for high horsepower motors and turbines is to install a reservoir of reasonable capacity and some means of continuously removing heat from the circulating lubricating oil system. This practice is found in machines performing service in the power, refining, petrochemical, pulp and paper and other industries.
The design of every system is unique to the conditions of that system. Every design represents a compromise among a number of elements, including available utilities, ambient conditions, cost, anticipated needs, machine mechanical condition and others. The nature of a compromise is that it generally does not completely satisfy all of the elements. Furthermore, some of the elements change over time. One result is that the lubricating oil cooling system can become, under some conditions, limited and unable to achieve the desired oil temperature.
Heat Transfer in Lube Oil Cooling
Purposes of lubricating oil include reducing rolling and sliding friction in bearings, providing a seal/fluid barrier to prevent mass transport from inboard (process) environment to the outboard environment and removing unwanted heat from the bearing zone.
The suitability of lubricating oil to perform its tasks depends on its thermophysical properties at point-of-use. The user can control one important property—viscosity—by adjusting lubricating oil temperature.
In practice, temperature control can be challenging. Lubricating oil systems that support large rotating equipment are subjected to variable (and, generally large) heat rate inputs. Sufficient heat must be removed so that oil returned to service is within a desired temperature range. That range is rather narrow. The means for removing heat from the oil is generally limited and the oil has poor heat transfer characteristics. Heat rejection and temperature control, while related, are two distinct topics. Problems arise when they are casually treated as one topic.
Lubricating oil systems supporting large machinery incorporate heat exchangers for heat rejection and temperature control. Enthalpy is extracted from the lubricating oil by heat transfer through the exchanger wall. The extraction rate is a function of the resistance to heat transfer, the heat transfer surface area, and the temperature driving force. The generally recognized thermal transport relation is:
The energy balance equations for the two constituent streams (lubricating oil and coolant) are given as:
The resulting three equations allow the user to quantify the performance of the heat exchanger and predict the lubricant oil temperature returning to the machine. To use the equations, however, the user must have a good value for U, the overall heat transfer coefficient. The coefficient is composed of five components that make a significant contribution to the result:
In lubricating oil systems, estimation of one of these components—hH, the oil side heat transfer (film resistance) coefficient—can make the difference between an accurate and inaccurate prediction. The film resistance coefficient represents the resistance to heat transfer from the bulk flowing stream to the wall of the heat exchanger. It reflects the complex interactions of heat, mass and momentum transport.
Considerable study of this physical phenomenon has been made over the last 70 years. To date, it has defied rigorous modeling. Rather, good progress has been made by means of semi-empirical predictions. These predictions utilize well-known Dimensionless Numbers, including:
All of the dimensionless numbers above, with the exception of the Prandtl number, have components that characterize mass transport. Heat transfer is facilitated in turbulent flow, and hindered in laminar flow. Reduced thickness of the boundary layer adjacent to the heat transfer surface wall and greater mixing in the bulk stream are consequences of turbulent mass transport. These result in improved heat transfer.
The semi-empirical predictions in today’s models make use of the relationship between mass flow conditions and the film resistance coefficient. It is in the modeled relationship between mass transport and heat transfer that the cautionary issue arises. Lubricating oils are specially formulated for their viscosity properties. As a result, generalized correlations which predict mass, heat and momentum transport phenomena for hydrocarbon streams can be inaccurate when used on lubricating oils. Generalized hydrocarbon correlations are used to calculate the components of dimensionless numbers and, in turn, to predict the film resistance coefficient. The generalized correlations are not sensitive to the specialized nature of lubricating oils.
Users today have a choice among a number of popular, commercially-available computer simulation programs to assist in analysis of lube oil systems. Properties and conditions can be entered into the programs, and heat exchanger sizing and performance predicted. As an additional aid, libraries of pre-determined properties are available as integral parts of the programs.
Prediction of heat removal from lubricating oil can be a challenge to commercially-available models. The thermophysical properties of lubricating oil create a situation where the results of the computer programs can be misleadingly optimistic. Four commercial programs were utilized in the design of the case study. All four gave predictions, with lube oil, of higher heat transfer than was actually observed in the field.
It was also noted that the use of lube oil properties from the libraries associated with the commercial programs introduced further inaccuracies. For sufficient accuracy in designing the projects of the case study, it was necessary to both use a purpose-built program and user-defined properties specific to the lube oil used in the project.
The result of the compounded inaccuracies is a significant difference from what would actually be observed in a real-world project.
This situation represents many facilities installed at this time. Designers, using commercial tools, have sized equipment to meet anticipated needs. If those tools give overly-optimistic predictions, the facilities will be undersized.
The second consideration is the possible impact of the temporary system on the permanent coolers. By lowering the temperature in the reservoir, the mean temperature difference (upon which the permanent, plant coolers operate) is reduced. Actual operating information indicate that, while MTD on the permanent coolers was reduced, the combined effect of the temporary exchanger and the permanent exchangers resulted in net increase in heat removal, and lower temperature lube oil return to the machines.
Allegheny Energy retained Aggreko to provide temporary, supplemental, lube oil cooling systems for the No. 1 and No. 2 turbines at the Pleasants Power Station. The results of that effort, utilizing the approach described above, are reported in this case study.
Each of the two 700 MW turbines at the Pleasants Power Plant in Willow Island, W.Va. has integrated lube oil reservoirs with coolers. For a number of reasons, the return oil temperature during periods of high ambient conditions approaches the alarm setting. Allegheny desired to eliminate this threat to the turbine bearings and potential reduced output.
The permanent coolers are incorporated into the reservoir, and not easily available for modification. Also, this problem exists only during a short period of the year (mid-summer). Thus, a solution based on temporary equipment had merit. During initial discussions, Allegheny and Aggreko agreed that the solution should be based on cooling a circulating slip-stream on the turbine lube oil reservoirs. Neither party wished to disturb the lube oil flow to and from the machine.
Allegheny desired that suction and return on the reservoir should be established from the top of the reservoir. This would minimize the potential for accidental spillage of lubricating oil. Also, the contents of the reservoir would not be at risk during installation, operation, and decommissioning of the temporary cooling system. This introduced some hydraulics challenges to the design.
It was agreed that Allegheny plant cooling water could be used instead of temporary cooling equipment (portable chiller or cooling tower). This would have some effect on the plant cooling water system hydraulics. However, the decision reduced project complexity and cost. Allegheny had, prior to initiation of discussions, installed stub-outs with valves in their plant cooling water supply and return systems in the area of the lube oil reservoirs. These existing stub-outs facilitated rapid installation of the project and made use of plant cooling water possible.
An important step in the design of the temporary project was the prediction of heat transfer performance. Four commercially available design models were set up for the conditions of the project and exchanger performance predictions made. Additionally, an in-house developed model that had shown good performance prediction with heavy oils was used. The results have been summarized in Table 1.
As can be seen, reliance upon the popular commercial models would have resulted in the design of a modification that was considerably undersized for the actual needs of the facility. A sketch of the temporary modifications is given in Figure 1.
Each temporary system was designed to reject about 2 MMBtuH from the lube oil reservoirs to the plant cooling water system. This was expected to result in lube oil temperatures to the bearings of less than 110 F during periods of high ambient temperatures. These results were achieved (Table 2).
For each lube oil reservoir, two circulation circuits were installed. The lube oil circuit included hydrocarbon pumps, which took suction from a dipleg extended into the oil from the top of the reservoir, forced it through the shell side of an exchanger and returned it back to the reservoir by means of another dipleg entering from the top of the reservoir.
The cooling water circuit conducted water from the plant CWS, through the tube side of an exchanger, and back to the plant CWR. Plant circulating pumps provided the driving force; no temporary supplement was used.
Installation and Commissioning
Aggreko provided the design, equipment and commodities and site advisory support. Allegheny provided installation labor. The project was needed soon after the decision was made to proceed as hot weather was predicted. The short schedule could only be achieved by use of temporary equipment, pre-work scoping and innovative engineering. In the interest of schedule, Aggreko proposed, and Alleghany agreed, to use braided, stainless steel flexible hose for the lubricating oil circuit. This material could be obtained rapidly and installed more easily than hard pipe.
The possibility of introducing solids contamination coincident with modifications is quite high. Construction activities, particulates in the area of construction and contamination of equipment and commodities are all normal conditions of projects. Regardless, contamination represents a threat to the oil quality, and thus, the bearings.
To avoid contamination from these sources, specific actions were taken. These included using new, braided stainless steel hose instead of pipe. This eliminated contamination associated with pipe fitting and welding, and also scale associated with hard pipe. Also used was specialty equipment cleaning procedures. Even though the major equipment used on the project had been cleaned to industry standards, it was all cleaned to a specification developed for this project. This eliminated any surface contamination, fine rust and so on. As a result, the installation had clean oil-contacting surfaces when commissioned. The project contributed no contamination to the lube oil circuit.
Both temporary cooling systems (supporting No. 1 turbine and No. 2 turbine reservoirs) were brought online in two steps. In each case, oil circulation was established with one pump in operation. The system was allowed to stabilize before the second pump was started.
The actual performance of the project is compared with the expected performance, developed during the design phase, in Table 2.
An analysis of operational data indicates that the incremental heat removal during one pump operation was approximately 1 MMBtuH. Incremental heat removal during two-pump operation was observed to be 1.8 MMBtuH. Compared to the design heat removal, as indicated on the turbine manufacturer’s exchanger sheets, these represent, respectively, 5 percent and 10 percent of the design duty. However, the reduction of the reservoir temperature over a relatively short period of time (20 F in less than two hours) indicates that the incremental heat removal is a much larger percentage of the total heat load.
For both turbines, the incremental cooling provided by the circulation from only one pump was able to reduce the reservoir temperature significantly. Incremental oil circulation, as established by the second pump, in each case increased heat removal and provided cooler oil to the bearings. For both reservoirs, oil circulation with one pump was sufficient to bring the oil system into the desired operating range.
The decision to use Allegheny plant cooling water as the cooling medium was demonstrated to be correct. Plant cooling water temperature and available flow rate provided a heat sink sufficient to achieve the project objective. In addition, no noticeable adverse effect to downstream equipment that utilizes plant cooling water was observed.
Modifications to lubricating oil systems for the purpose of supplemental cooling can be achieved without increasing risk to the rotating equipment serviced by the oil. Sizing the supplemental project to achieve the desired result requires specialty expertise. Commercially available tools can give misleadingly optimistic estimates of heat transfer performance. Systems designed on the basis of this misleading information can be undersized. Utilizing the practices set forth in this paper, commercial installations can be made that will achieve the process purposes intended.
Authors: Larry Denk, P.E. is a principal engineer and Michael Karlin is a process engineer with Aggreko Process Services based in Houston. Daniel Earnest is project manager and Loren Bartlett is plant engineer with Allegheny Energy’s Pleasants Power Station.