Avoiding Component Failure in Industrial Refrigeration Systems

Transcription

1 Avoiding Componen Failure in Indusrial Refrigeraion Sysems By Tim Kroeger, segmen markeing manager indusrial refrigeraion, Asia Pacific & India The aricle caegorises and gives examples of ypical componen failures in indusrial refrigeraion sysems. I liss he roo causes and describes how o avoid hem. Focus is on liquid hammer, safe ho gas defros and general refrigeraion piping. Opimum resuls by correc selecion, inegraion and operaion indusrialrefrigeraion.danfoss.com

2 1. Liquid hammer 1.1 Definiion Liquid hammer is a erm used o describe a pressure spike caused when a fluid in moion is forced o sop or change direcion suddenly (momenum change) 1. Liquid hammer can occur when a valve closes suddenly a he end of a pipeline sysem, and a pressure wave propagaes in he pipe. Liquid Hammer can also occur when liquid is suddenly acceleraed and hen forced o change momenum. Liquid hammer is also called hydraulic shock. Examples of liquid hammer include he following: Sudden deceleraion. High impac forces can occur when a valve is closed quickly. Sudden acceleraion. Mos commonly encounered during ho gas defros iniiaion and erminaion. Also referred o as Vapour Propelled Liquid Slug (VPLS). CIS (Condensaion-Induced Hydraulic Shock). Sudden condensaion of vapour bubbles in highly sub-cooled (>20K) liquid. No covered in his aricle. 1.2 Sudden deceleraion An esimaion of he pressure spike occurring during he sudden deceleraion of a liquid in moion can be aemped by visualising a moving column or slug of liquid in a pipe ahead of a valve or an armaure, see figure 1. V The force generaed by he deceleraion can be calculaed as wih: p pressure [Pa] A cross secion [m 2 ] m mass [kg] a acceleraion [m/s 2 ] Δv velociy change [m/s] ime of deceleraion [s] or: wih: F = p x A = m x a = F = p x A = m x a = p = L x ρ x Δv m x Δv L x A x ρ x Δv L lengh of pipe [m] ρ cross secion [kg/m 3 ] A Example A solenoid valve is insalled in a 20 mere sraigh run in a slighly undersized liquid line (velociy 3 ms -1 ). Refrigeran is ammonia (densiy 600 kg/m 3 ). The closing ime of he solenoid valve is esimaed a 20 ms. Esimaion of pressure surge V L x ρ x v 20 m x 600 kgm p = = -3 x 3 ms s Figure 1: Moving liquid in pipe When his slug of liquid is suddenly sopped, a force builds up agains he cross secion of he valve or armaure which equals he force resuling from he rae of change of momenum. m kgm -3 ms -1 kgm m p = 1,800,000 = 1.8 x 10 6 s s 2 m 2 = 1.8 x 10 5 Pa =18 bar This is an over pressure, i.e. above he already exising liquid pressure of he sysem (mos commonly he condensing pressure). This could easily resul in exceeding he maximum safe operaing pressure of componenry. Hydraulic shock caused by deceleraion can be observed by banging or knocking of he valve and over ime can lead o sress fracures. 1 ) Wikipedia, definiion of fluid hammer.

3 Recommendaions If he solenoid valve is insalled in a HP liquid line (e.g. feed line ino a LP vessel) and is no pilo operaed, he closing ime will be raher shor, due o he high pressure differenial across he valve. In order o minimise he inernal volume of liquid line valve saions, he use of compac valve saions designed for high pressure applicaions such as he Danfoss ICF should be considered (see figure 5). Therefore, one should consider a proporional liquid level conrol wih a moorised valve, see figure 2. M Figure 5: Danfoss ICF Valves Saion Figure 2: Moorised valve for liquid level conrol Excessive pipe runs beween solenoid valve and meering device (hand regulaing valve) should be avoided for overfeed evaporaors. When he solenoid valve is acivaed, he sudden inrush of liquid can cause hammering a he hand regulaing valve, see figure Sudden acceleraion Acceleraed liquid ofen occurs when high pressure vapour mees low o medium pressure liquid. When he acceleraed liquid mees an obsrucion, which in urn resuls in a change of momenum, pressure spikes much higher han hose caused by deceleraed liquid occur. This is called Vapour Propelled Liquid Slugging (VPLS). VPLS can be classified ino wo forms of liquid slugs: saic and dynamic 2. A saic slug is liquid rapped in a porion of he piping, compleely filling i. A dynamic slug is a fron of liquid, propelled by vapour. In a 6 pipe, as lile as 1 ½ of liquid can lead o a dynamic slug, see figure 6. Figure 3: Avoid excessive pipe runs I should also be ensured ha all liquid line valve saions are sized correcly by aking ino consideraion he overfeed raio. An oversized liquid feed valve can lead o excessive liquid velociy. During commissioning, care mus be aken ha all regulaing valves are adjused correcly. A wide open regulaing valve can also cause excessive velociy, see figure 4. Saic Dynamic Figure 6: Saic and dynamic liquid slugging Figure 4: Effec of oversized liquid feed valves 2 ) Shelon and Jacobi e alera, 1997

4 2. Ho gas defros and VPLS The roo cause of VPLS is ofen an incorrecly designed and operaed ho gas defros sysem. For example, if he ho gas defros pressure is conrolled by a muli-purpose valve in he we reurn line, and he ho gas is herefore injeced ino he boom of he evaporaor coil, here will be a high risk of acceleraed liquid. Also, if a single sep solenoid valve is used in he we reurn line, here will be a sudden pressure equalisaion afer defros erminaion. This can also lead o acceleraed liquid. As he velociies are much higher han in he previous example for deceleraed liquid, pressure spikes of up o 250 bar can occur, see figure 7. In he correcly designed ho gas defros sysem, he ho gas is inroduced ino he evaporaor during defros from he op, hus eliminaing he risk of acceleraed liquid a defros iniiaion. The defros pressure is conrolled by a separae pressure conrol valve, enabling a purpose designed wosep solenoid valve wih minimal pressure drop (usually ho gas powered) o be used in he we reurn line. These valves are available on he marke oday (e.g. Danfoss ICLX). See figure 8. NC NO Figure 7: Commonly designed ho gas defros sysem (no recommended) Figure 8: Moving liquid in pipe Operaion Liquid We reurn Ho gas Fan Timer [s] Commen Cooling Open Open Closed On N/A Conrolled by hermosa Defros sar Closed Open Closed On Closed Closing Closed Off Closed Closed Closed Off Pump down ime depending on evporaor size Closing ime for ho gas powered sucion valve - ICLX Allowing remaining liquid o sele a boom of evaporaor Defros Closed Closed Open Off Depends on applicaion Closed Closed Closed Off Drip ime Defros sop Closed Open Closed Off Sucion valve opens slowly. Saic cooling o freeze droples Open Open Closed Off Saic cooling o freeze droples Cooling Open Open Closed On N/A Conrolled by hermosa Table 1: Safe and efficien ho gas defros sequence

5 3. Piping design and layou ASME 3 code for pressure piping sipulaes ha consideraion mus be given o he expansion of liquid refrigeran rapped in or beween closed valves and a means mus be provided o preven over pressure. Examples of such occurrences are shown in figure 9, e.g. liquid rapped beween wo shu-off valves, beween a nonreurn valve and a solenoid valve or even beween a nonreurn valve and a back pressure dependen regulaor. In all hese cases, suiable measure o preven over pressure (such as relief valves) mus be used. Figure 9: Examples of rapped liquid 4. Summary In conclusion, i mus be said ha when inroducing auomaic conrols for sysems previously conrolled by manual means, i is imperaive o selec he correc componens for he purpose a hand, inegrae hese componens in a safe way ino properly designed indusrial refrigeraion sysems and operae hem in an efficien safe way o obain opimum resuls.

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