What’s the Difference Between Low and High Pressure?

1 48 August 2004 CleanerTimesxactly what are pressure washing, water jetting, and low and highpressure? It is a good question. The distinction is not all that clear. Everyonehas a different definition, including the current WaterJet Technology Associa-tion (WJTA) board May 2004, Cleaner Timesput the question on their website and got this answerfor What is High pressure ? from Donnie O Neill of Covington, Louisiana: I think the term high pressure should be any pressure above 2500 psi. But thatalso depends on whom you re talking to and about what. A homeowner workingwith a pressure washer [would say] anything around 2000 psi.

2 A person workingin a plant may say 10,000 psi is high pressure . Pressurized water has grown from essentially garden hoses to 50,000 psimanually held field units. Descriptive terms like Water Blasting and HydroBlasting are very common in this the Eighties, contractors were hired to remove dirt, mildew, paint, asphalt, andwhatever. Owners/operators were specifying, Use [high pressure ] water blasting. The response bids had a wide spread in price. Contractors would show up with a high pressure water blaster that often turned out to be low pressure and sprayedwater all over the place without getting the desired cleaning results.

3 The ownerswould throw up their hands and say, We are going back to traditional methods scraping, sand blasting, etc. because water doesn t work. There was no self-regu-lation within the industry. People with better equipment got low-balled. Standard Terms, Relatively SpeakingThe following definitions are taken from WJTA s Recommended Practices For The Useof Manually Operated High pressure Waterjetting Equipment , November 2002, p. 34 High pressure water cleaning the use of high pressure water,with or without the addition of other liquids or solid particles, to removeunwanted matter from various surfaces, and where the pump pressure isbetween 5,000 psi (340 bar) and 30,000 psi (2,040 bar).

4 EWhat s the Difference BetweenLow and High Pressure? by Lydia M. Frenzel, more information circle 251 CleanerTimes August 2004 49 For more information circle 136 Where the term high pressure is used without further qualifica-tion it is considered to describejets being used at pressures below30,000 psi (2,041 bar). pressure cleaning Theuse of pressurized water, with orwithout the addition of other liq-uids or solid particles, to removeunwanted matter from various sur-faces, and where the pump pres-sure is below 5,000 psi (340 bar). Ultra high pressure watercleaning The use of high pressurewater, with or without the additionof other liquids or solid particles, toremove unwanted matter from varioussurfaces, and where the pump pres-sure exceeds 30,000 psi (2,041 bar).

5 SSPC-NACE committee membershad to make compromises withinthe painting industry. The dryabrasive blasting industry did notwant blasting to be used in ourdefinitions. Blasting was to be re-served for inclusion of abrasives, sowe could not use the common waterblasting or hydro blasting separated the issues of includ-ing solids and not including started with ultra-high at 25,000psi and have since moved to 30,000 Abrasive Blast Cleaning(WAB) is the paint industry s termfor including abrasives or solids,in low to ultra-high pressure watercleaning. WAB covers abrasives ad-ded to water streams or water ad-ded to abrasive streams, in amountsranging from mostly abrasive witha little water to mostly water witha little abrasive.

6 Generic termstodescribe specific air/water/abrasiveblast cleaning methods are: watershroud or wet-head blasting, wetblasting, low volume water abra-sive blasting, and slurry terms to describe specificwater/abrasiveblast cleaningmethods are: slurry blasting, abra-sive waterjetting (AWJ),or abrasiveinjected waterjetting/blasting (AIWJ).Additional DefinitionsTerms were defined for thecoatings industry in NACE No. 5-50 August 2004 CleanerTimesSSPC SP-12, Surface Preparationand Cleaning of Metals by Water-jetting Prior to Recoating, firstpublished in 1994, to reduce con-fusion and to assure that when acompany asked for high pressure the contractors delivered the equip-ment and NO.

7 5- SSPC SP-12 in-cludes these definitions: Water Cleaning (WC):Use of pres-surized water discharged from anozzle to remove unwanted matterfrom a Jetting Water:Water ofsufficient purity and quality thatit does not impose additional contaminants on the surface beingcleaned and does not contain sed-iments or other impurities that aredestructive to the proper function-ing of waterjetting Water Cleaning (HPWC):Water cleaning performed atpressures from 34 70 MPa (5,000 10,000 psig).Low- pressure Water Cleaning (LPWC):Water cleaning performed atpressures less than 34 MPa (5,000psig). This is also called powerwashing or pressure washing.

8 The distinction Between low and high must be made on amarket-availability basis. Pumpsup to 3000 psi can be found in mostany industrial supply store. Thereare many manufacturers offeringconsumer and commercial clean-ing packages. At around 5,000 psi,the number of pump manufactur-ers contracts significantly. The5,000 psi pump packages have thesafeguards and features that ex-tend up to the 20 25,000 psi sectors may use the terms low, medium, or high at otherlevels or from NACE No. 5 Waterjetting (WJ): Use of stan-dard jetting water discharged froma nozzle at pressures of 70 MPa(10,000 psig) or greater to prepare asurface for coating or uses a pressurizedstream of water with a velocitythat is greater than 340 m/s (1,100ft/s) when exiting the Waterjetting (HPWJ): Waterjetting performed at pres-sures from 70 to 210 MPa (10,000 30,000 psig).

9 Ultrahigh- pressure Waterjetting(UHP WJ): Waterjetting performed atpressures above 210 MPa (30,000 psig). The WJTA doesn t define jet-ting. Dr. David Summers has anentire chapter in WaterjettingTechnology dedicated to Whatis a waterjet? without defining jet. Summers cleaning tablestarts at 70 bar (1000 psi) and 20lpm (5 gpm). The NACE/SSPC task group thinks of jet as morethan a pressurized stream comingout of a nozzle. The NACE-SSPC group bases their distinction onthe velocity of the water stream at1100 ft/sec the speed of soundin air. As early as 1974, papers atthe WJTA and BHRA symposiumswere defining the shape of awaterjet structure in engineeringterms of an initial core, a Adams delivers all three!

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