Rotary Piston versus Turbine Flow Meters – Application strengths

We’re often asked why we recommend our VFF Rotary Piston device for chemical injection and, in particular, against the traditional turbine flowmeter.  Here are 8 specific advantages of the VFF over turbine flowmeters:

  1. Low Flow The lowest flow measurable by a VFF rotary piston positive displacement flowmeter is two orders lower, especially if the viscosity is greater than water. For example, the LF03 at just 10cP will measure down to 0.025 l/hr – that’s a teaspoon every 12 minutes. A small turbine, even at 1cP, it’s optimum viscosity, peters out at 0.03 l/min – over 70 times higher.
  2. Pulsing Flow Customers tend to use pulsing pumps for chemical injection and even with pulsation dampers turbine meter blades will tend to over spin and cause the flow meters to read high if you are measuring total flow. Pulsation dampers are set at a pressure higher than the line pressure and are only guaranteed to reduce the pulsation to within ±10% at best of line pressure. If the line pressure changes at all during operation, then pulsation dampers are useless as they can only be set for one pressure. In our experience PD meters are ideal as they track the pulse and will read accurately for total flow even if no dampers are fitted.
  3. Display stabilisation The second problem with pulsing flow on a turbine meter is that the flow rate indication or 4-20mA output will range up and down at a frequency slightly out of synchronisation with the actual flow. To stabilise this reading rolling average electronics are often used. However, this can have two negative effects: firstly, the rolling average function, which is software based, will increase the response time on a control loop, the effect being that any changes in flow will take longer to be corrected. Secondly, you can easily get the control loop constantly ranging up and down chasing the flow meter reading but never quite catch up. These could be expensive for the client with wasted chemical. PD meters do not require the rolling average function as their reading while moving with the pulsing flow will be accurate.
  4. Viscosity effects Turbine meters are inferential devices: even small changes in viscosity due to ambient air temperature changes will be sufficient to affect the accuracy. In addition, if the viscosity is above 1 cSt then the lower end of the flow range will be less accurate and some of the measurement range will be lost. With chemical injection applications offshore it is inevitable that through the year there will be a wide range of ambient air temperatures and viscosity changes are very likely. PD meters are not affected by viscosity changes; in fact, if the viscosity increases their accuracy gets even better as the slippage past the gears or piston reduces. In our experience the only application where a turbine meter may be used for chemical injection is Methanol as it has a very low viscosity of 0.6cSt at 25°C and much higher injection rates. Even so, VFF meters are often selected for Methanol.
  5. Maintenance If something fails in service in a turbine meter the whole unit must be removed from the line. With a PD meter the meter body can be left in the line and the top cover removed to replace the internals.
  6. No straight lengths Turbine meters typically require a minimum of 10 diameters upstream and 5 diameters downstream of the flow meter, increasing installation space requirements.  In most configurations of piping prior to the meter much more is needed. Positive displacement flow meters do not require straight lengths.
  7. Non-standard connections There is a design issue if the turbine assembly cannot slide through the connector inside diameter. If it can’t physically pass through then there has to be at least one if not usually two adaptors. For example, Autoclave Engineers fittings can have 5mm id tubes. It is not possible for most turbine assemblies to be made this small.
  8. Range ability Most turbine flowmeters have flow ranges limited by bearing friction, hydrodynamic influences like viscosity and sensor drag. A ratio of maximum to minimum flow is typically 10:1, maybe higher of the viscosity is low. In the LF03 example given in point 1, the turndown ratio is 720:1. Furthermore, in the LF series the rotors and chambers are interchangeable. The LF15 and LF05 will drop into the body of an LF body previously holding an LF03 chamber. If the flow rate is set to decrease over the years of installation, then a new chamber can extend the life of the unit. Any future iterations of the VFF will use the same size ensuring the LF02 or LF01 will extend the range even further down.

If there are any doubts then please consult the specialists.

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