LPG Autogas Handbook - Rutherford Equipment

Effect of VaporizationNet Positive Suction HeadRefer to Figure 2 for an illustration of the effects of vaporizationon an underground and above ground tank applications. As theliquid level drops in the tank, the vapor above expands and itspressure drops. Immediately, the liquid in the tank begins toboil, creating vapor bubbles. Liquid entering the dip tube/inletpiping carries some of the bubbles with it. Each restriction in thepump inlet piping drops the pressure of the liquid-vapor mixture,causing the vapor bubbles to expand and causing more boilingand more vapor bubbles to form. In addition, a significant pressurereduction for lifting applications is due merely to the change inelevation from the tank fluid level to the pump inlet. All the vaporbubbles entering the pump are rapidly collapsed back into liquidwhen they move to the discharge side of the pump. LPG vaporoccupies much more space when vaporized than in the liquidform. Therefore, a significant volume reduction occurs whenevera vapor bubble is collapsed on the discharge side of the pump.This explains why a pumping system never delivers as much actualliquid as the pump flow rating would indicate.Net Positive Suction Head Required (NPSHR) is the minimum inletpressure required at the pump inlet to avoid excessive cavitation.The Net Positive Suction Head Available (NPSHA) must be greaterthan required NPSHR to prevent pressure at some regions in thepump suction area from dropping below the liquid’s vapor pressure.If the inlet pressure is lower than the vapor pressure, vapor bubbleswill form in the liquid. As the bubbles travel through the pumpingchamber to the higher discharge pressure region, the bubblesimplode and release high amounts of energy, commonly calledcavitation. The collapse of the vapor bubbles causes pressure spikes,resulting in noise, vibration and damaged hardware.In addition, vapor bubbles reduce a pump’s capacity since thepumping chamber volume is being filled with a mixture of vapor andliquid. The vapor bubbles will occupy a volume, which under normalconditions, is filled by liquid. Although PD pumps are less susceptibleto vapor lock than a centrifugal pump, under severe conditions, PDpumps will vapor lock as well.NPSH becomes more of a concern in certain applications such aspumping from an underground tank, cold weather pumping andrunning a pump at motor speed.All typical propane transfer applications operate with some degree ofvapor in the suction line. As observed in LPG fluid transfer, the pumpwill deliver more flow when recirculating to the supply tank versusdelivering to a cylinder or receiving tank.This reduced flow rate is due to the increased amount of vapor in thesuction line during the delivery operation. As the supply tank liquidlevel drops, the tank pressure drops and the liquid boils, creatingvapor bubbles. The vapor bubbles travel along the inlet piping to thepump. As the fluid moves through the pumping chamber, the highpressure discharge fluid collapses the vapor bubbles. The implosionof the vapor bubbles creates the noise that characterizes cavitation.NPSH required is normally determined with room temperaturewater per the Hydraulic Institute’s standards. However, a typicalLPG installation operates with less NPSHA than when testing withwater. Other pump manufacturers, both centrifugal and positivedisplacement (PD), have reported the same findings.These general facts have been established from years of experienceregarding PD pumps for LPG applications:1. PD pumps can handle some entrained vapors without adverseeffect to pump service life.2. A flooded inlet condition NORMALLY provides sufficient headfor a PD pump to operate. However, poor inlet piping can andwill cause many pump problems.3. Pumps handling LPG will operate with less NPSHA than wouldbe required for water.4. Even pumps with properly designed suction piping may have anegative NPSHA.5. Use a larger size pump to compensate for the lower efficiencyat cold temperatures.6Figure 26. Keep discharge pressures as low as possible by using vaporreturn lines.

Cold Weather PumpingAs the ambient temperature and product temperature drop below40 F (4 C), a noticeable reduction in pump capacity is observed.An installation located in a cold weather region must considerthis capacity reduction when selecting the proper pump size. Asshown in Figure 3, pump efficiencies are significantly lower in acold environment.There are several guidelines that can minimize the effects of coldweather pumping:1. Avoid fittings in the inlet such as tees, globe valves, plugvalves, angle valves, check valves and standard port ballvalves that collectively exceed 2 PSI inlet pressure drop.2. Use as few fittings on the inlet piping as possible.3. Use an inlet pipe one size larger than the pump inlet port.Figure 34. Install the pump as far below the source/supply tank aspossible.5. Install the pump as close to the source/supply tank aspossible and avoid long horizontal runs.6. Avoid low spots in the inlet piping where vapor bubbles canaccumulate.7. Use a larger size pump to compensate for the lower efficiencyat cold temperatures.8. Keep discharge pressures as low as possible by using vaporreturn lines.9. Slope or angle the inlet piping from the supply tank downtoward the pump. This will allow entrained vapor bubbles tomove back toward the supply tank.Note: Pumping butane is more difficult than pumping propane, itis similar to pumping propane at a colder temperature. The samepump and piping set at a given temperature will perform worse onbutane than on propane.7

2. Tank Design and SelectionAbove Ground TanksMany Autogas refueling sites in North America utilize aboveThis vaporization imposes a rather rigid limit on the maximumground tanks, often 1,000 gallon (3,785 liter) capacity because theydelivery rate that can be achieved with ANY size tank. Theare readily available. However, experience has shown that largermaximum withdrawal limit in standard, non Autogas LPG tanks istanks, 2,000 gallon (7,570 liter) capacity, provide better overallabout 2½% of the tank capacity per minute for pumps mountedoperation than smaller 1,000 gallon tanks. When considering thebelow the tank. However, for high-pressure Autogas systems thatsize tank to be used, users need to determine their annual usageoften utilize smaller tanks, a 2% withdrawal limit is more realisticand then size the tank accordingly. From a pumping perspective,and practical. Therefore, the supply tank should be at least 50larger tanks will provide better conditions than smaller.times larger than the nominal flow-per-minute rating of the pump.It is also good practice to elevate the tanks so that the bottom ofFor example, if a tank has a capacity of 2000 gallons (7,570the tank is at least 4-6” above the pump inlet – higher would beliters), the pump should be rated for a maximum of 40 GPM (150better. (Refer to NFPA 58 prior to installation to confirm maximumLPM). Under-sizing a tank can be a costly mistake. It will causeallowable height.) Elevated tanks allow room for piping transitioninadequate delivery rates due to liquid vaporization and boiling,and still allow for level or downward sloping pipe from the tankpump damage due to cavitation and potentially lost business iftoward the pump. It also allows better valve and componentthe system cannot deliver product. When sizing a tank, error onaccess for maintenance and emergency response.the side of caution to ensure a trouble-free system.Tank SizeMany Autogas system integrators prefer to use 1,000 gallonThe size of the supply tank has a significant effect on overall systemmaximum withdrawal rate on a 1,000 gallon tank would beperformance. As soon as liquid is removed from a supply tank,limited to 20 GPM (75 LPM). Over-speeding the pump or usingthe remaining liquid in the tank will begin to vaporize. As liquida larger pump will have little or no effect once this barrier hasis removed, the pressure in the tank drops slightly. To maintainbeen reached. If a larger pump is used or a higher flow rate isequilibrium, the fluid will begin to vaporize or boil. If liquid isattempted, the liquid will vaporize in the inlet piping and causeremoved too rapidly, significant vaporization and boiling willpump cavitation and potentially vapor lock the pump. Oftenoccur. Liquid in the tank will actually cool, reducing the equilibriumwhen multiple refueling hoses are used, flow rates higher than 20temperature and increasing the vaporization rate.GPM are required. If that is the case, then it is good practice to use(3,785 liter) tanks, following the limits mentioned about, thelarger supply tanks.NOTES:VAPORELIMINATORMETER &ELECTRICALCONTROLSCABINET1.ALWAYS MAKE SURE THE LIQUID LINE TOPUMP IS PITCHED SLIGHTLY DOWN TOWARDSPUMP2.ALWAYS CHECK WITH LOCAL FIRE CHIEFBEFORE INSTALLATION OF UNITREMOTE SHUTOFFCABLE PULL TO CLOSEDOME1075NO R &PUMP81" PUMPBYPASSLIQUIDLINE1" BALLVALVE1-1/4" XHSTRAINER1-1/4" GOOD1-1/2" BETTER2"BESTSCH 80 PUMP SUCTIONFigure 4INTERNAL VALVE, QUICK ACTINGWITH EXCESS FLOW PROTECTION,AUTOMATIC THERMAL CLOSURE,AND REMOTE CLOSING SYSTEMBASE CHANNEL

Underground TanksThis section presents practical solutions to the problem ofattempting to lift LPG from underground tanks with a pump. Thedrawings in this guide portray only those components pertinent tothe topic being discussed.All the information presented in previous sections on FluidProperties and Effects of Vaporization and above ground tankdesign apply to pumping from Underground Tanks. Vaporization of fluid at the pump inlet will cause reduced flow. Temperature and propane/butane mixture ratios will alsoaffect delivery rates. Reducing the temperature of the fluid or increasing thepercentage of butane in the fluid will reduce the vaporpressure in the storage tank. Lower vapor pressure results in the formation of larger vaporbubbles since gas volume is inversely proportional to pressure. Minimize the Number of Fittings Every fitting, valve, and piece of straight piping causes apressure drop. Use a minimum number of fittings on theinlet side of the pump. Eliminate all possible elbows in theinlet piping by moving the pump so that they will not benecessary. Sizing the inlet piping one or two pipe sizes largerthan the inlet of the pump will reduce the overall pressureloss of the inlet piping. Strainers Suction strainers should not normally be used on undergroundtank installations as they create a significant pressure dropduring priming and normal operation. Contaminates in a tankshould settle to the bottom. In a properly designed system,the end of the dip tube should be placed two to three inches(five to eight centimeters) above the bottom of the tank. InTesting has shown that 1,000 gallon underground tanks loose NPSHapplications with known high levels of contaminates, install aat about 95-110 PSI tank pressure. Lower pressures often causestrainer that is one or two sizes larger than the pump inlet topumps to vapor lock, regardless of the technology used. In viewminimize the pressure drop.of these operating issues, 1,000 gallon tanks are not practical forunderground installations as they will only yield approximately200-300 gallons of useable capacity. Hence, 1,000 gallonunderground tanks are not recommended.When pumping from an underground tank, the change in elevationfrom the fluid level in the tank to the inlet of the pump will causesignificant vaporization of the fluid in the inlet piping. This happensbecause the pump reduces the pressure below the liquid vaporpressure which causes the fluid to vaporize in the inlet pipe. For thisreason alone, it is impossible to prevent vaporization at the inletof the pump for an underground tank installation. However, thereare many things that can be done to minimize these effects. Thefollowing bullet points will help to optimize the performance ofan underground tank application. Refer to figure 2 as a guide to atypical underground tank application. Vapor Removal During Startup Sliding-vane positive-displacement pumps, such as Blackmer’s Install Vapor Excess Flow Valve The vapor excess flow valve provides a path to return thevapors to the tank during startup. When a liquid flow ofapproximately three GPM is established, the vapor excess flowvalve will close. When piping the return line from the vaporexcess flow valve to the tank, ensure that there are no lowspots where liquid can collect. If liquid is present in the vaporreturn line, excessive pressure will be required to push vaporsout of the pump. Pipe the vapor return line to the vapor spacein the tank, NOT to the liquid space of the tank or to the inletof the pump. Install the pump directly over the tank Keep the pump as close to the ground as possible and use theminimum number of fittings. Use inlet piping one or two sizes larger than the pump inletconnection and keep the inlet velocity below 3 ft/sec.LGL pumps, are capable of moving vapor as well as liquid. ThisFor additional information on proper installation of pumps onenables the pump to “self-prime” and establish liquid flow.Underground Tanks, please refer to Blackmer’s Underground TankHowever, when a pump is operating without liquid, excessiveApplication Guide, Bulletin 500-002.heat and wear will result. (See the Bypass Valve Piping sectionregarding the Ebsray VRS - Vapor Removal System.) Inlet Piping Length Keep the inlet piping as short as possible. Install the pump directly over the tank and as close to theground as possible. This will eliminate the overall volume ofvapor that the pump must remove during startup.9

3. Tank to Pump Inlet DesignNOMinimize Line LossesPipe and fitting size are critical to pump suction, thus a properlydesigned system will perform better. In general, lower pressurelosses will require less power to drive a pump and result in reducedstart up cavitation, vapor lock and unintended excess flow valveclosures. Typically, pressure drop from tank to pump inlet should beless than 2 psi.Installing piping or fittings that are too small will increasethe system differential pressure, cause vaporization and inletrestrictions which will seriously degrade system performance.Figure 5In general, tank liquid supply valves and pump supply piping2. The pump should always be at a lower level than the tank’sand components should be equal to, or greater in size than thepump inlet size. Eliminate any unneeded fittings, particularly onthe liquid supply line, as this is where most of the pressure losseslowest liquid level. Install the pump to eliminate any low spotsor loops in piping. Figure 5and vaporization will occur. Use low restriction fittings and valves,3. Shield the pump and inlet piping from direct sunlight to reducewhich will minimize pressure losses and always ensure all strainerpriming issues. Painting the suction lines white will help reduceelements are clean. Minimize use of elbows, tees and branches onthe radiant heat absorption. Note: aluminum paint absorbspump inlet piping. Use 45 elbows where possible. Use of pluggedinfrared heat at a higher rate than pastel colors or white.tees as elbows is not recommended unless absolutely necessaryfor additional or future connections. While minimizing the lengthof pump inlet piping is desirable, making it as “as close as possible”to the tank outlet can result in excessive restrictions if extra elbowsand/or fittings are used.4. Suction line pressure losses must be less than 2 psi (0.14 bar).To minimize pressure losses in the suction line, use lowrestriction type valves and fittings. Also, long sweep radiuselbows are preferred. If possible, changes in pipe size shouldbe made vertically, where the pipe first exits the tank. ThisSuction Line Pipingtype of installation allows vapor to flow up and the line sizeOne of the most important considerations for a proper installationelbows in the inlet stream, to no more than a total of 180 ofis the suction line piping. A properly designed suction line willbends (i.e., 2 x 90 elbows, or 1 x 90 and 2 x 45 elbows).then remains constant to the pump. Where possible, limitminimize vapor formation by limiting the line restrictions andpreventing vapor pocket formations. In light of the tendency of LPGliquid to vaporize, the design of the intake piping plays a critical rolein the efficiency of every system.Following are some good design practices for LPG suction lineinstallations. Adhering to these guidelines will promote longerpump service life.Storage TankLargeDiameterUse White PaintBall ValveAs HighAs PossibleBall ValveBall ValveClean Strainer10D1. Inlet piping from the tank to the pump should be as short asDpossible, with the following conditions:A. The pipe where connected to the pump inlet, should bethe full inlet diameter, straight pipe, with no fittings orcomponents which could cause turbulence of vaporization.B. This final section of straight pipe should be a minimumof 10 times the pump inlet diameter (i.e., 2" inlet 20" ofstraight pipe), and threaded or flanged directly to the pumpinlet, with no additional fittings. Figure 610LongRadius EllKeep ShortFigure 65. The nearest fitting or strainer should be a minimum of 10pipe diameters from the pump inlet. Where possible, limitelbow connections to a maximum of 3 elbows. Use full port¼ turn valves for component isolation. It is a good practiceto install 3 full port ball valves, isolating the pump, bypass,

strainer and flex hose for maintenance, with a bleeder valvemade as close as possible to the internal valve and vertically ininstalled for section depressurization. These are the mostorientation, if possible. Figure 10.likely components to be serviced, and can thus be easilyaccessed with minimal loss of product.10. Long inlet lines should be avoided, even when they are solarge that they have practically no friction loss. At night orInlet/Outlet Diagnostic Guagesin cold weather, the liquid will cool. Then, as the day warmsor, as sunlight shines on the pipes, the cool liquid will beheated, causing some liquid to be vaporized. This vapor willdecrease the pump’s flow rate and increase the noise andvibration. To minimize this problem, inlet lines should belevel or sloped upward toward the supply tank so vapors canflow back into the tank. Figure 11.11. Avoid up-across and down pipe loops where vapors canaccumulate. Figure 5.12. When supplying two or more pumps from the same liquidopening of the tank, ensure that the same length of pipe withFigure 7the same number of bends is supplied to each pump. The6. It is highly recommended that inlet and outlet gauges bemanifold needs to be adequately sized to provide sufficient flowinstalled for diagnostic use and for confirming differentialto both pumps. This will minimize the possibility of the flow pathpressure to adjust bypass pressure. Gauges may be installed“favoring” one pump over the other, which could result in onedirectly on gauge ports on the pump. Figure 7. Good qualitypump being starved of liquid when both pumps are running.glycerin filled gauges, of at least 400 psi (27.5 bar) will yield theNote: If both pumps are to run at the same time, the commonbest results. Is

Blackmer LG and LGL Sliding Vane Pumps These ductile iron pumps, available in 1 in. to 4 in. sizes, are all UL listed for LPG, Butane and Anhydrous Ammonia Service. Models are available for motor fueling, cylinder filling, vaporizers, general transfer and truck loading/offloading. Blackmer/Ebsray Regenerative Turbine PumpsFile Size: 2MB