Filter Press Troubleshooter

Transcription

1 Filter Press Trubleshter Filter Clth is Blinding Blinding, when little r n filtrate cmes ut and the back pressure ges up in a hurry, the cake fails t frm r is wet. This is usually a prcess prblem. Smewhere upstream a change tk place. One f the first things t ask is, When did it happen? Befre r after the media was cleaned r changed? If befre, it is prbably an upstream prcess prblem. If the slurry is changed making it mre difficult t dewater the slutin may be t add a bdy feed. The bdy feed can be anything frm diatmaceus earth t grund up vlcanic rck. It is added t the slurry t add bulk t it. A secnd ptin is t g t thinner cakes that can be dewatered. This means machining dwn the packing surface n the frames r recessed plates, r, changing the plate stack. Yu get a smaller batch size but the driest ne. Althugh the cakes are thinner, additinal chambers can be added in the new space generated by the thinner plate stack. If pssible, try t crrect any variables upstream that cause a wet cake. Precipitatin, the sneaky enemy, happens ver time where slids in the filtrate cme ut f slutin. Like lime in a teakettle. Check the hand f the media, if it is stiffer than when new, it s a gd indicatin f precipitatin. Yu may even get pieces f precipitate t crumble ff int yur hand. Anther classic is il in a water mixture r vice versa. If ne gets int the media it stps the ther frm cming thrugh, perid. The inlet pressure is brught up t fast. When this happens the slids are immediately cmpressed int the media interstices blinding them. It s like a night club where every ne tries t get thrugh ne small dr at the same time, in a panic. Lg jam; nbdy, nthing gets thrugh. The symptms are a thin layer f dry cake next t the media and slurry everywhere else. Take a lk at the pump. If it has an air perated diaphragm pump lk fr a ball valve n the air inlet. Instant 100 psi int the pump and instant slurry inlet pressure f 100 psi int the press. The less experienced the peratr, the mre likely the valve ges wide pen, right nw. It is almst always best t bring the pressure up slwly. This gradually builds up the cake and allws the filtrate t pass thrugh it and the media. The pressure builds up at the very end. Timers can be used t bring up the pressure gradually. On centrifugal pumps a blck and bypass can be installed.

2 If the pump is a diaphragm pump always suggest a surge suppressr fr it. The surge suppressr reduces the hydraulic hammer when the pump switches diaphragms. Withut it the line pressure can g frm in a fractin f a secnd. The whle press bangs and jumps. Nt gd. The inlet pressure is t high. In sme instances the nature f the slurry is such that it just wn t filter abve a certain pressure. Yu have t maintain a pressure based n the slurry characteristics, nt a pump curve. Flcculants are sensitive. When they surrund small particles and frm clumps, the water arund thse particles is released. The clumps g against the media, the water ges thrugh, and all is gd. Nw, hw can this be messed up? Fr starters t much flcculent can be added t the slurry. If a little is gd, right? This frms a g against the media blinding it. Hmmn, less filtrate is cming ut, crank up the pump pressure. Which really drives the flcculent int the media blinding it. If this happens n a belt press the flcculent lks like tiny tapewrms hanging n the rller side. If this happens n a filter press the clths have t be cleaned r changed. We d have a cleaner fr it, cntact yur NFM sales rep. Wrk n better peratr training. Precats are catings f diatmaceus earth r grund vlcanic rck, put n the filter media prir t the main filtratin event. A precat acts as a filter media. They are usually mixed with a separate liquid cmpatible with the slurry and then pumped int the press filling frm the bttm f the frame r recessed plate. An average thickness is 1/8. The press has t be set up with the drains shut at the bttm and pen at the tp. This lets the precat evenly fill the chamber frm bttm t tp. The precat is recirculated until n trace f it appears in the filtrate. Then the slurry is pumped in, fills the chambers, the lwer drain is pened and yu filter. Can yu imagine all the pprtunities f messing this up? There are lts. The peratr can put in the wrng precat. Hey, it all lks the same t me. The whle amunt is dumped in at nce instead f fed slwly. Hey lk, all the chambers at ne end are full f precat and there is nthing at the ther end. What bttm drain valve? The precat fills the bttm inch r s f each chamber. On each ne f these it appears the precat went in fine and then blam, des the media blind ff! Yu can be sure that the peratr will clean the press by remving the filter aide and starting up again. But what abut the slurry that had gtten int the media where there was n precat n the prir run? New piping? Yes it can. As plants change their prcesses they may decide t change their piping t better utilize their vessels and hlding tanks. All that s needed is t put in a new run f pipe frm the utlet f the press up a stry r tw and acrss the plant. Nw that pr filtrate must nt nly g thrugh the cake and ut the press but up 10 and acrss 20. And what if the slurry has a lt f viscsity? Yu get a wet cake, but nt because f the media. Yu may suggest that either the piping be changed r an inline pump be installed. Just be sure it desn t pull a real vacuum n the press. On metal plates, there can be precipitates filling in the drainage pyramids leaving a flat drainage panel. Desn t wrk. Fr a few times it can be sandblasted clean. Chemical attack n the drainage pyramids gives yu the same net result. The clth may have been calendared t much at the mill.

3 Particulate Bleeds thrugh Filter Clth Bleed-thrugh, the ppsite f blinding. When the slids r parts f the slids pass thrugh the media with the filtrate. Again, the 1 st thing t d is ask has the prcess been changed and had the clth been OK until then? Operatrs in need f retraining? Here are sme causes- Precat They put in the wrng precat. Sund familiar? If the wrng precat is really carse stuff the fines may g right thrugh. The whle amunt was dumped in and ended up in ne end f the press letting all the slurry fines blast thrugh the clths at the far end f the press. Bttm drain valve is pen all the time again. All the precat is in the bttm half f the press letting the slurry fines sht thru the media in the upper half. ph-this gets yu ging like few ther things in waste treatment. When certain metals are treated as waste their ph is adjusted t a narrw range that frces them t cme ut f slutin. If the ph is either t lw, acidic, r t high, alkaline, the metals g back int slutin and turn up in the filtrate. Fewer things get peratrs mre excited than the thught f hazardus wastes ging int a sanitary sewer. Big fines. The cause is usually an upset in the chemicals added t keep the ph in the crrect range. T many r t little were added. Human errr, bad ph meter, bad injectr pump. Pumps & Flcculent- Anther prblem with flcculants can be the pump. They are sensitive t shear. Centrifugal pumps have lts f shear and s d diaphragm pumps with n surge suppressr (at higher pressure ranges). Media- It can be trn r punctured. This can be caused by using smething sharp t knck ut the cake. A plate panel can be brken and tear thrugh the media. Hw d yu find it? Yu can peel the clths back and lk fr the drainage panel with cake n it. It s a dead give away. The clths can be munted wrng with a clth eye spanning frm a recess t a drain eye. Clth r thread may be incmpatible with yur peratin. The threads are chemically attacked. They lse their strength and may disslve. Operatrs will ntice this right away. Plates- If there is a scratch, guge r lw spt in the packing surface between a drain eye and the chamber the slids take a shrtcut. Lk in the individual drain eyes t see where the slids first appear. That shuld be the ne. In a thrugh washing filterpress, the drainage eyes must all be in the same alignment. There is usually a mark n the sides f the plates (and frame) indicating the sequence f the plates. Yu start with the mark n the fixed head plate. It may be single mark, if it is a recessed plate the next plate has tw, then ne. They alternate. If it is a plate and frame press the fixed head plate will usually have a single mark, the frame tw marks and the next plate three. A sequence f If a plate r frame is reversed the prting sequence can really be messed up. The bttm right crner is a feed fr a frame and then a drain fr the plate next t it. By the way, it is nt uncmmn fr a custmer t buy a used filter press and get plates and/r frames frm anther sent with it.

4 Filterpress is Leaking Use a set f clths with latexed packing surfaces. They are very effective. See if the current clths and/r plates have a precipitate build up. If they d the plate stack will be wider at the bttm than the tp. This is what yu will see- With the press clsed under pressure yu will see the tp f the plates literally arc. High at the ends & lw in the middle. Get yur tape ut and measure all fur crners f the plate pack. They shuld all be the same. Want t take bets? Re-measure with the clths ut. The measurements shuld still be equal. Want t take anther bet? All the leaks cme frm the tp f the press. The media has t be cleaned t remve the precipitate (acid washed?) r replaced and the lwer packing surfaces have t be cleaned. Then remeasure. The hidden prblem (again), backpressure. When the press is running the cake makes a darn effective seal against the media. But there isn t ne n the drain panel side, unless yu latex the packing surface. The greater the head the mre the press will leak. Recessed plates have thrust buttns mlded int the panels. The buttns are the same height as the packing surfaces. When the press is clsed they line up with ne anther minimizing any panel mvement. Can yu bw r break a panel n a recessed plate? There is a way. T decrease the capacity f a recessed press peratrs will mve the slide head plate frm the end f the press t smewhere in the middle. On larger plates, even with thrust buttns, yu can get a delta P f 100+ punds per square inch f panel. It will g between the buttns and packing surfaces, destrying the slide head plate. The remedy is simple, a dummy plate is needed. A dummy plate is a plate inserted int the press next t the back side f the slide head plate. It has its wn handles and is thick enugh t take up the unbalanced pressure. It is abut twice as thick as a recessed plate. They can be inserted anywhere in the press allwing If yu want t run the whle press there is usually enugh rm t keep it between the slide head and slide head plate. See yur Natinal Filter Media sales engineer abut getting ne. Diaphragm Plates- These are usually alternated with regular plates in the press. In time the diaphragms will wear ut and leak. An inability t hld pressure int the diaphragms is a gd indicatin f a leak. Be careful hw ht the press is with diaphragm plates. As the slurry temperature ges up the amunt f pressure in the diaphragm shuld be turned dwn. Cnsult yur Natinal Filter Media sales engineer.

5 Catastrphic leaks happen with a cmbinatin f factrs A. Pump pressure int the press is s high it literally pushes the plates and/r frames apart. The slurry vilently escapes frm the weakest pint. If yu think this desn t happen ften lk arund at the walls and ceiling where the press is. If there are plyprpylene frames in the press they will be bwed upwards in the middle and/r the tp brken ff t ne side. A psitive displacement pump, like a gear drive pump set t the wrng pressures, can d it. B. High temperatures will sften plyprpylene t where it begins t lse its tensile strength. Again, this is mst cmmn in a press with frames. It has the same net result as abve. Only this time it s ht t. There are different grades f plyprpylene plates that take higher temperatures and pressures. Yur Natinal Filter Media representative will be glad t qute them & slve the safety issue.. C. A & B plus a precipitate build up. D. The clsing device n the press desn t have enugh clsing pressure. It may have bad valves, pump, been designed undersized r ended up that way in the current applicatin. Maybe the peratr didn t crank up the hand pump enugh? If it is a mechanical failure Natinal Filter Media has access t a technician wh may be able t repair r replace it. He wuld knw hydraulics. E. Thermal shck t the plate pack can be caused when the press filters at higher temperatures and then wash water at a much clder temperature is pumped int the press. The press is then under pressure with tw different temperature gradients passing thrugh it. Plates r frames may crack. There is a chance that very cld hse wash water n ht plates r frames may als have the same result. Bring up the wash temperature. F. All the abve. When yu sell a plate pack be sure what A & B are- As thrugh as this truble shting guide is, there are always new challenges that cme up when perating a filter press. Call us if yu d have a prblem, we d like t help. Dave Sperry- Filter Press Specialist 630/ dsperry@nfm-filter.cm