Application of Lubricate slot and liquid buffer gasket seal at hot water work condition
Hot water work condition adopt conventional mechanical seal is absolutely not use hot water wash seal face directly,hot water must be cooling,fall the wash liquid's temperature under 65～82℃.
Usually,the project of wash seal at hot water work condition, haveAPI plan 23 and API Plan 21.Thereinto API plan 23 is educe hot water from seal box( stuffing box ) ,after through a hot exchange then to wash seal face.Hot exchange fall the hot water's temperature before re-inject into seal box to wash seal face.The drive of wash liquid circulation is supply from a inside pump deliver ring usually,also named as pump ring.
API plan 21 is educe the hot water from the exit of pump,through hot exchange to fall temperature,then inject into seal box and wash seal face.Generally speaking,API plan 21 is not effectively than API Plan 23, because it need cooling hot water from craft pipe line,usually need more big hot exchange at the same work condition,the advantage is wash liquid needn't pump deliver ring supply drive.
Besides ,can use at hot water work condition wash project as well as API plan 32.
This project API plan 32 is adopt a external low temperature clean soft water inject into seal box to wash seal face,it's wash pipe line is simple and reliable,but the disadvantage is wash water's wastage is big even dilute or cool the hot water at craft line,it's hard accept with much customer.Certainly,can adopt add cooling water out seal box to fall the hot water's temperature at seal box.
Hereinbefore wash project,all require seal box's hot water's temperature fall to the standard lever which the seal design can accept,make seal achieve a acceptable useful life;also require supply another line cool water,it's bring large cool water consume,furthermore,once intermit cool water at operate process to made the seal invalidation speedy.At the same time,those wash project also face the hot exchange、jacket box and other places dirty problem at the use process,it's reduce the reliability of seal system at practice operate process.
Following introduce two kinds seal manner are suit hot water work condition as well as needn't cool water:"Lube Groove"and"Hydropad"mechanical seal.
At a standard mechanical seal,between two seal face have a thin liquid film,in the process of produce and form steady liquid film,liquid's physics characteristic be at a critical state.when liquid through the seal face, pressure was reduce,temperature was increase.The cause of pressure reduce is difference pressure between inside and out side edge of seal face.,the cause of temperature increase is the liquid in liquid film come under shearing force and mechanical contact produce attrition hot between seal face.Liquid film avoid excessively big mechanical contact between seal face , thereby to reduce attrition hot and face attrition at seal face operate process.If liquid film boiloff near seal face edge,then mostly main seal face will lost support from liquid film,cause the seal lost efficacy soon.
"Lube groove"is a narrow groove chiseled at seal face along tangent direction,this groove can't through seal's in and out side .when liquid flow through
Seal face, this groove can reform liquid's pressure distributing in seal face.It's help to keep liquid film's stabilization between seal face and avoid liquid boiloff at liquid film.
Due to high boiloff pressure of hot water,easy to arouse celerity boiloff through seal face,so control the pressure at seal contact face is the key to solve problem.When seal face are parallelism,pressure in liquid film show linearity distributing basically,outside of seal face's seal box pressure is reduce to inside atmospheric pressure step by step.When liquid film's liquid pressure reduce to equal to liquid's boiloff pressure step by step,liquid film will boiloff.Through to open lube groove on seal face,form a relative high pressure area on seal face.The pressure distributing is from out side of seal face to inside of seal face show linearity descending.When through lube groove,the liquid pressure keep constant,then reduce show linearity from lube groove to inside of seal face.The form of pressure distribution's change limit the liquid film's boiloff at ring shape area be cover with lube groove.
Following is "lube groove"seal's application at hot water work condition:
max seal size:5inch(127millimeter)
max out edge line speed:66feed/second(20miter/second)
Highest rotate speed: 3000rpm
Lowest rotate speed:750rpm
Highest seal box pressure:15Bar face pressure
Highest seal box temperature:175C
Seal material:graphite,carborundum,horniness alloy
Commendatory pipe line project: API Plan11 or API Plan02
Tip:Lowest seal box pressure should keep above 2Bar.
Under ordinarily condition,in order convenient for manufacture,"lube groove normally cut at soft seal face.The determinately number and length depth of "lube groove" is decide by the size of seal face.
"HYDROPAD"is the groove or incision,cut at rotate or static seal face from edge to inside.The depth from one in thousand to 0.125inch(3millimeter)
At normal seal,seal face is even cirque shape.After open groove at seal face,mechanical and temperaturewill arouse transfiguration at cirque direction.In mechanical aspect,cutting groove weaken the seal face's part strength:In hot power aspect,groove make the different of lost speed rate along the seal face's cirque direction.This two factor cause seal face arise undulance transfiguration.Due to liquid film is very thin,wee transfiguration will bring big effect with liquid film's form or stability.
Undulance transfiguration is help to liquid film extension between seal face.Liquid film's thickness's constriction and enlargement create a pushing power made seal face separate.This liquid push power reduce the mechanical contact ,crate heat and seal face abrasion between seal face.The size of push power not only connect with seal face's undulance transfiguration,but also connect with seal's operate characteristic and physics characteristic.The relatively speed is also a important factor to create liquid push power.The speed is more fast ,the liquid push power will more strong.Liquid's characteristic, such as specific gravity,viscosity,steam pressure,create big effect with liquid all.Liquid's specific gravity and viscosity more small the liquid push power more small.Due to above characteristic.“liquid cushioning”can use hot water,light hydrocarbon and hydrofluoric acid etc medium.
To define wether need "liquid cushioning" should consider flowing factor :
1)Boiloff pressure:When seal box's pressure under liquid's boiloff pressure plus 25 Psi (means 1.7 Bar) ,should consider temperature factor at the same time.
2） Pressure difference:When inside and out side edge of seal face's pressure different is very low or very high(big than 250 Psi means 17 Bar),should consider adopt the stye of "liquid cushionind".Because of when liquid's pressure is very low,liquid's pressure can't conquer spring's power be action in seal face.,hard to form liquid film.Moreover,when liquid's pressure is very high,seal face endure high close liquid pressure,hard crate liquid film also.
3)liquid specific gravity:liquid specific gravity small than 0.52.
At present,"liquid cushioning"same as "lube groove"is cartridge seal .
Mechanical seal type use at hot water work condition compare at flowing table.
|
seal face type |
seal face structure type |
washing project |
Cooling water |
Pc
Seal box pressure |
|
tradition mechanical seal |
flat surface |
API Plan 21、23、 32 |
need |
without limit |
|
"lube groove"seal mechanical |
open close at soft seal face |
API Plan 02、11 |
need not |
Pc<Pv+2 bar且2<Pc<15Bar |
|
"liquid cushioning"seal mechanical |
open out edge open ended groove cut at arbitrary seal face |
API Plan 02、11 |
need not |
Pc<Pv+1.7bar或△Pc>17Bar |
Notation:Pc –seal box pressure;Pv –medium's boiloff pressure;△Pc –seal face's in and out edge pressure difference.
application example
At ShangHai SECCO company styrene project,one condensation pump's temperature is 1170C，entrance pressure is 1.6 Bar，exit pressure is 7.2 Bar，account the seal box's pressure is about 3.2 Bar，medium's steam pressure is 1.41 Bar，proportion is 0.951.
In original design, adopt API plan 23 project.This design need a suit of assistant pipeline system,hot exchange and need expend 0.7meter3/hour cooling water.We after studied and synthetically compare with owner,decide to adopt "lube groove"( QB Lube )stye seal face,washing project is API Plan 11.Can save about 30% equipment purchase cost,operate fee seal can save about 6000/year(just cooling water expend)
Due to "lube groove"mechanical seal need not cooling water,avoid seal lose efficacy as a result of cut water mistake operate,as well as due to hot exchange create dirty reduce the reliable of seal system.This type seal have 40 years history at Europe and Middle East.
Expect have much success application at hot water work condition,"lube groove"and "liquid cushioning"seal have abundance successfully operate at some special work condition,such as：liquefy gas,low temperature liquefy gas,light hydrocarbon,hydrofluoric acid and other medium.because it's substituted the dry gas seal at above special work condition,more over the purchase cost and operate cost,difficulty of operate all under dry gas seal very much,believe have more and more widely application in the future.
