Temperature & Pressure Rating

All Macnaught flow meters are designed to be completely safe under normal operating conditions. However, to ensure user’s safety, it is very important to select a flow meter that will operate within the process pressure and temperature conditions at all times. Allowances should be made for any potential ‘spikes’ in pressure (e.g. as a result of sudden valve closures or as the pump initially starts). If the system pressure is expected to reach the meters maximum rating it may be necessary to incorporate a pressure relief valve into the system. Macnaught’s Technical Support Team is available to provide advice in this regard. There are a number of factors which influence the pressure and temperature ratings of a flow meter during operation:

  • Presence of flanged ports causes a reduction of the pressure rating
  • Other modifications to the materials properties

Temperature rating:

  • Operational parameters such as the limitations of an attached LCD register and batteries
  • Coefficient of thermal expansion or• Other modifications to the materials properties

When factors combine, the individual effects could amplify and pose a safety risk. We therefore recommend seeking expert advice from Macnaught’s Technical Support Team before final selection of the appropriate meter.

Rotor Type

While Macnaught Positive Displacement Flow Meters are capable of processing a very wide range of fluid viscosities, the viscosity of fluids still need to be considered to aid the selection process. This value should always be taken at the applications operating temperature and flow rate. Note that typical fluid technical data sheets are not usually stated at operating conditions, so further research may be required to determine this information. In cases where the fluid is non-Newtonian, additional allowances may be required to compensate for changes to viscosity between static and dynamic situations. The higher the fluids viscosity the greater the pressure drop it will cause. As shown in Figure 7, the maximum flow rate will need to be reduced as the viscosity increases. Please contact Macnaught if your require further assistance. Alternatively, as shown in Figure 6, Macnaught offers High Viscosity (HV) rotors that have been modified to alleviate this pressure drop and still offer extended flow ranges. As a general rule if the viscosity is >1000cP it is recommended to use HV rotors, but it is also important to look for notes on minimum viscosity as this can also affect the meters minimum flow rate.