Filtration of heat transfer fluids increases efficiency

1. PECIAL S SPECIAL REPORT Filtration of heat transfer fluids increases efficiency W ith p ro p erly fun ctio n in g eq uip m en t b ein g o f utm o st im p o rtan ce, h o t o il filtratio n p ro ves to b e vital in in creasin g p ro d uctivity an d keep in g co n tam in an ts o ut. By Zak Shum s,Liq uid Pro cess System s In c. The sources of problems H fluid between hot oil boilers and reactor vessels, tanks, molds, calenders, extrud- ers, or heat exchangers. The fluids are subjected to temperatures ranging from 300°F to 750°F for heating applications in the chemical, plastics, rubber, petro- chemical, pharmaceutical, pulp and paper, fiber, and food industries. ot oils or heat transfer fluids are used for the indirect heating of processes by circulating the heated Proper maintenance of the system will control and retard degradation of heat transfer fluids. Some of the contributors to degradation are exposure to oxygen (air), low velocity of the fluid through the heating chamber and piping, im- proper heater selection, and operating the system at temperatures above manufac- turer’s recommended maximum temper- ature. The by-products of degradation are Th is filter system filters h eat tran sfer o ils up to 650°F an d 100 p sig system p ressu re. 98 • Engineer's Digest • September 1997

2. sludge and coke due to carbon formation. Contaminants that may be circulating in the system are pipe slag, mill scale, dirt, and dust accumulated in the system dur- ing installation or maintenance. As the amount of contamination increases in the system, the fluid undergoes drastic prop- erty changes that affect the heat transfer capability of the overall system. Problems caused by contaminants are wear of rotating components (pump im- pellers, gears and shafts, mechanical seals, valve stems, etc.), reduced capabil- ity of heaters and heat exchangers, in- creased viscosity of the oil, and increased energy consumption. Analysis Heat transfer fluid should be analyzed at least once a year as part of proper maintenance. Depending on the usage, the analysis might be performed quar- terly or even monthly to determine the condition of the fluid and compare it with specifications from the material safety data sheet for the oil. The analysis should include measure- ment of specific gravity, total acid, vis- cosity, insolubles, and flash point of the fluid. (For more details, see the accompa- nying box, “Heat transfer liquid test analysis,” p. 100.) Analysis of the fluid provides a snap- shot of the sample’s condition. It is criti- cal to take a live sample of the fluid. That is, the sample should be collected while the system is operating at operating tem- perature. Care must be taken while draw- ing the sample. One should wear protec- tive clothing including heat resistant H o t o il filters use cartrid ges (sh o w n ) w ith excellen t d irt h o ld in g cap acity, an d th ey o ffer easy d isp o sal. gloves, face shield, and eye protection. The valve should be opened slowly, and the fluid should be collected in a clean metal container. The sample should be drawn close to the discharge of the process pump where the turbulence is maximum. After collecting the sample, one should seal the container as soon as possible. Depending on the level of degradation, the system may have to be drained, flushed, and replenished with filtered or new fluid. Preventative action, including filtration and isolation of the fluid from contacting the atmospheric air, should be incorporated to preserve the integrity of the fluid and the system. September 1997 • Engineer's Digest • 99

3. PECIAL S SPECIAL REPORT Filtration methods A traditional method for filtration of hot oils has been to incorporate a strainer before the system pump. Strainers are de- signed to protect a piece of equipment such as a pump, valve, or flow meter. They trap particles, preventing the parti- cles from entering the pump and eventu- ally the system. A strainer must be cleaned regularly to prevent cavitation, which causes mechanical seal failure or magnetic de-coupling. Another filtration method is to install a filter in either a side stream or full flow arrangement. For both arrangements, the filter consists of a filter housing with a basket made of stainless steel, with per- forations designed to trap fine particles. For side stream installation, the inlet of the filter is installed close to the dis- charge of the pump. The fluid is diverted through the filter, cleaned, and dis- charged to the suction side of the same pump or to a low-pressure return line. The most effective filtration is a forced flow and side stream system arrangement that incorporates a pump and filter de- signed for high temperature use. This ap- proach employs its own filter pump to di- vert 5 to 10% of the process flow contin- uously through the filter as the heating system is operating. All of the fluid passes through the filter at least 15 to 20 times per day. The inlet of the filter pump should be plumbed into the discharge piping close to the process pump to take advantage of the turbulence in the piping which keeps the solids in suspension. The dirty fluid is forced through the filter and the clean fluid is discharged into the same process Heat transfer liquid test analysis Sp ecific gravity.G reater th an th e o rigin al liq uid m ean s o th er m aterials are p resen t, in d i- catin g p resen ce o f lo w o r h igh b o ilers an d co n - tam in atio n . M o istu re. W ater h as lo w so lub ility in m o st h eat tran sfer liq uid s excep t glyco l. T AN . Also kn o w n as T o tal Acid N um b er o r N eutralizatio n N um b er . Th e Acid /B ase titratio n d etects stro n g an d w eak acid s in th e fluid . In so lub les. In d icates th e am o un t o f in o r- gan ic m aterials such as p ip e slag, san d , co n - structio n d eb ris, an d co ke carried b y th e fluid . H igh /lo w b o ilers. W h en h eated to h igh tem - p eratures, certain m o lecular b o n d s b egin to b reak o r d egrad e. H igh an d lo w b o ilers d e- crease h eat tran sfer efficien cy an d th erm al stab ility. Visco sity. Fluid flo w ch aracteristics p er un it tim e in d icates th erm al d egrad atio n . G C scan . Th is test gives th e sign ature o f th e d egrad atio n co m p o n en ts o f h eat tran sfer liq - uid an d o ften d etects co n tam in an ts. Flash p o in t. Th e C levelan d O p en C up test p ro vid es a m ean s o f d etectin g fire/flash p o in t o f liq uid . line at a pressure higher than the system pressure or to the return pipeline down- stream from the process pump. Components It is important to ensure that the mate- rial of construction of all the components on the filtration system are suitable for high temperature use. Many filter hous- ings use “O” ring elastomer seals which are not safe for high temperature opera- 100 • Engineer's Digest • September 1997

4. tion because they can lose strength or dissolve in the heat transfer fluid. It is important to consider the replace- ment of the contaminated filter cartridge. The initial cartridge should be rated to filter 100- to 50-micron size particles and gradually reduced down to filter 25-mi- cron size particles. Filtering the fluid to a micron size less than 25 may remove addi- tives from the oil and affect the perfor- mance of the fluid and system. Depth filters using glass-fiber-wound fil- ter cartridge elements have proven to be most effective in this type of filtration sys- tem. They will with- stand temperatures up to 700°F, have excel- lent dirt holding ca- pacity, and are eco- nomical. A typical filtration system should include a pump, filter, controls, isolation valves, and safety accessories. ED Reader feedback: Your comments help us produce a magazine that is useful to you. Please rate this article by circling your re- action on the Reader Service Card. Very helpful—Circle 381 Somewhat helpful—Circle 382 Not helpful—Circle 383 Liquid Process Sys- tems, Inc. provides side stream filtration sys- tems designed to oper- ate up to 650°F for hot oils. Contact Shums at (704) 821- 1115 for more informa- tion or circle 346. Zak CIRCLE CARD NO 559 September 1997 • Engineer's Digest • 101