FAQ: LPG Applications

Blackmer FAQ: LPG Applications

• Can I use an SGL Series Sliding Vane Pump to pump propane?

  The SGL Series Sliding Vane Pumps are designed with a cavitation suppression liner to support pumping liquefied gases.  The LGL Series Sliding Vane Pumps are used for pumping propane for UL-listed applications. SGL Series Sliding Vane Pumps are the preferred pumps wherever UL is not required. 

• Can I use a different mechanical seal with an LGL Series Sliding Vane Pump?

  Since LGL Series Sliding Vane Pumps are UL certified for LP-Gas and Anhydrous Ammonia applications, the mechanical seal cannot be changed to a different type of seal. If a different seal is required and the application is not under UL jurisdiction, then the SGL Series Sliding Vane Pumps are a preferred option, since they have multiple mechanical seal options available.

• If a dispenser is mounted remotely from the tank, where should the pump be mounted - closer to the tank or the dispenser?

  The pump should always be mounted close to the supply tank. The pressure loss of the additional piping on the pump discharge will have a minor affect to the operation compared to creating additional cavitation if it is included in the pump inlet connections. Proper pump selection, bypass valve, line sizing, and system layout can accommodate multiple hoses and dispensers, sometimes exceeding 150 feet (43 meters). It is recommended to select a pump with 10-20% more flow rate capacity than the combined flow rate of all dispensers/hoses in the system. This will ensure sufficient volume in the discharge piping for each hose when in simultaneous use.

• Should the pump discharge line size and vapor return size from the pump be changed if the dispenser is mounted a distance from the pump?

  Yes. Line sizing is important when the dispenser is mounted remotely from the tank and pump. Typically, the following should be considered:

  • Dispenser 25' (7.5 meters) or less from pump – Pipe size from pump to dispenser: ¾" minimum and ½"  minimum vapor return line
  • Dispenser 25-50' (7.5 – 15 meters) from pump – Pipe size from pump to dispenser: 1" minimum and ¾" minimum vapor return line
  • Dispenser 50-100' (15 – 30 meters) from pump – Pipe size from pump to dispenser: 1.25" minimum and 1" minimum vapor return line
  • Dispenser greater than 100' (15 meters) from pump – Pipe size from pump to dispenser: 2" minimum and 1.5" minimum vapor return line

• Will downstream components affect overall performance?

  Yes. Pressure drops over pipe distance just like suction piping. All fittings in the system (elbows, unions, valves, filters, etc.) effectively add additional length. Many Autogas vehicles now require a minimum of 125 psi (8.6 bar) differential pressure to reliably fill under all ambient conditions. So a system set for 135 psi (9.3 bar) at the bypass valve by the pump, may only deliver 110 psi (7.6 bar) at the dispenser 100 feet (30 meters) away with multiple couplings, unions, valves and fittings in between. To minimize friction loss over distance, the use of wide radius “sweep” elbows and welded pipe will yield better performance than the disturbance from threaded NPT connection and screwed elbows. As in suction piping, use ONLY full port ball valves.

• What happens if the sliding vane pump sits idle for long periods, especially when the dispenser is remotely mounted?

  It is very common for most of the liquid to vaporize in both the discharge and suction piping when the sliding vane pump shuts down for a period of time. This can cause problems on startup, as well as introduce vapor to the dispenser meter, making it work harder and/or vapor lock. To help resolve this problem, use internal valves on all tank openings and keep them closed when not in use. The excess flow mechanism in the valve will act as an internal pressure relief, and naturally may open to allow vapor to return, but it will slow the rate at which the fuel vaporizes. The use of the Blackmer and Ebsray bypass valves with the constant bleed feature will help speed pump priming. If there is an excessive amount of vapor to be removed, the Ebsray RV18VRS bypass valve will allow the vapor to be removed even faster. Removing the vapors in the pumping system quickly will drop the pressure to allow liquid from the tank to quickly supply the pump.

• What can be done if a pump frequently vapor locks when using tank internal valves on the pump supply line and bypass valve return line – especially after sitting idle for some time?

   

  The use of excess flow valves in ALL system openings in the tank (pump supply, bypass return and vapor return from the dispenser meter) is recommend for extra safety, especially if the valves only open when the pump is turned on by the dispenser, and kept closed when not in use. However, when this is done, a time delay relay MUST be installed on the motor starter circuit so that the internal valves fully open allowing for vapor to return to the tank unrestricted, and a full supply of liquid to prime the pump. Typically, this is done by starting the pump 5-10 seconds after the internal valve is opened, depending on the distance from the tank to the pump and from the pump to the dispenser. Some dispenser manufacturers offer station control panels, which integrate the dispenser, internal valve and pump motor control, and have this time delay function built in.

  Blackmer Bypass Valve “How it Works” Animation

   

• What can be done if an application requires a 7.5 hp motor, but the site only has single phase power? Is there a solution for this?

  Yes – the use of a variable frequency drive (VFD) work very well to make reliable 3 phase power from typical 208/230 single phase power. VFD’s are an inexpensive option if the drive will be mounted inside a building. However, note that it is important for motors over 3 hp, that the VFD drive be properly sized for single phase input. VFD’s should NEVER be mounted outside unless they are in an enclosure specifically rated for drives and inverters. These enclosures usually have ventilation and cooling fans and a different type of paint finish to reflect and insulate from heat. Consult an experienced VFD/Pump distributor or Installer, or a pump VFD control panel manufacturer. Failure to properly size and install a VFD, or a rotary converter, may result in premature drive or motor failure. Please note: the motor selected must be an “Inverter Duty” or “VFD rated” motor.

• Are there any special considerations for redundant pumps, so in case of failure there is a backup pump?

  Many fleets are used to redundancy, as a fleet simply cannot operate very long without refueling capability. So a second “back-up” pump is a good idea. However, several things should be considered.

  1. Both pumps should have identical plumbing, equal in all respects for both suction side and discharge side piping. It is critical that the suction piping does not “favor” one pump over the other.
  2. It is a good idea to have a bypass for each pump but they should be installed in such a way, that one can be isolated from the other, but can work with either pump.
  3. It is good practice to occasionally run both pumps to ensure that the secondary pump will run as expected when needed. This is commonly done with a timer or auto switching relay. However, if both pumps are operated for equal amounts of time, then one can reasonable expect that if one fails from normal wear, the secondary pump may fail shortly after from equal wear. Therefore, it is a good idea to limit the secondary back-up pump to operate only 25% of the time, just enough to ensure that it is in good working order when needed.

• If two or more pumps are needed for a multiple dispenser application, can the same bypass valve be used? Or should each have a separate bypass? If both pumps are supplying the same line to the dispensers, ...

  Using separate bypass valves is necessary, IF each pump is supplying a separate and not interconnected discharge pipe to different dispensers.

  If two or more pumps are feeding the same discharge pipe to the dispensers, then using multiple bypass valves can be difficult to properly set. The bypass valves, when set to the same differential pressure, can have hysteresis, or can have their pressures fighting each other. This can result in pulsations and poor differential pressure control. So it is a good idea to have a 5 psi (.34 bar) difference between the setting of each valve. Additionally, it can be beneficial to use the next size larger bypass valves. If a single bypass valve is used, then it must be rated for a minimum of the total flow capacity of all pumps it is supporting. However, if only one pump is operating and the valve is too large, then it may not adequately control pressures. So it is often best to have several hoses on a single supply manifold, and each pump supporting a separate supply manifold.

  If two or more pumps are supplying the same discharge pipe to a dispenser island, it is a good practice to install a full port back check on the discharge side of each pump, after the pipe running to the bypass valve. This will keep one pump from back flowing the other and also prevent the pumps fighting each other.

• Can a pump with a flow rate greater than the meter it is supplying be used? i.e., a 24 gpm (90 L/min) Autogas pump with a meter rated for 18 gpm (68 L/min) flow rate?

  Possibly. Often a meter runs better with a pump which is rated for slightly MORE than the meter rating, than it will with a pump rated for less. This helps ensure steady pressure and flow through the metering chamber. A meter’s rated flow rate is usually GREATER than the application it is filling. Therefore, while a dispenser is rated for 18 gpm (68 L/min), when filling cylinders or Autogas applications, it will rarely, if ever, flow more than 12-15 gpm (45-58 L/min) maximum. The container/ cylinder being filled, and the limits of the OPD valve and system design, are what ultimately governs flow rate, not the pump or the meter. When calibrating the meter, the flow should be limited by the inlet valve on the prover to within the NCWM rating of the meter. The bypass valve used with the pump MUST be sized to accommodate both full flow (when nozzle is closed) and any excess flow, not required by the dispensing application.

• What is NPSHr of Blackmer LGL, SGL, or CRL Sliding Vane Pumps?

  NPSH questions are common, because NPSHr vs. NPSHa is a common acceptance criteria for pumps. 

  • NPSHa is an allowance provided by the system.
  • NPSHr is an absolute pressure measurement and corresponds to a pump’s mechanical efficiency at 95% of the design flow rate.
  • In theory, if NPSHa > NPSHr, cavitation is limited within a pump.

  For high vapor pressure fluids and liquefied gases (i.e. propane, condensate, etc), a NPSH comparison is not applicable because these fluids are a multiphase mixture of liquid and gas.  Vapor is always present in these systems, making cavitation unavoidable.  Instead, the appropriate criteria is includes limited pressure loss within suction piping, maximum withdraw rate relative to size of source vessel and additional constraints detailed in the Blackmer Liquified Gas Handbook (500-001).
  
  

  Blackmer Cavitation Series Sliding Vane Pumps (CRL, LGL, SGL) are well suited for operation under continuous cavitation and vapor/liquid mixtures.

  • The Cavitation Series Vane Pumps leverage the unique cavitation suppression technology of Blackmer. This video and white paper provide more information.
  • The LGL Series Vane Pumps are backed by an unconditional 2-year performance assurance guarantee, underscoring our confidence in operating under continuous cavitation.
  • A comprehensive guide to pumping LPG is provided in the Blackmer Liquified Gas Handbook (500-001).

  In summary, an NPSH comparison is useful for the proper selection of pumps to avoid excessive cavitation.  However an NPSH comparison should not be used as acceptance criteria for applications where vapor is expected regardless of the NPSHa condition.