For "sticking" process reasons, I want to implement PVST (Partial Valve Stroke Test) across the whole spectrum of ON/ OFF valves (Safety and Non-safety related) with triggering through FF Positioners.

How many FF Outputs are allowed to be put on one FF Segment?

Our site specs indicate that only 2 Control Loops can be put on 1 segment, and therefore if I have 70 On/ Off valves I would need 35 Segments?

If one Positioner draws only 15mA loaded and my spurs are short (most valves are in the same location), then can I not implement 8 outputs maximum onto the 1 Segment? (No spare contingency.)

Thanks in advance.
BR,
Graham

 

First thing to do is evaluate the current carrying capacity of the home run cable supplying the spur. You can do this by obtaining the cable resistance of the home run going to the power supply. You also need to know the lowest voltage required among the devices and deduct it from the power supply output voltage to get the voltage used by the cable (Vcable = Vps - Vlowest). From this we get the load capacity of the home run: Icable = Vspur/Rcable, and from there know the number of devices it can support: N = Icable/Idevice. Idevice is the current drawn per device, not the total. If the devices don't have identical current draws, then get their average to get Idevice.

If this number satisfies your total requirement (N > 8 + existing), then it is a good indication to proceed. Be careful not to be too exact when getting the Vlowest among the devices. The device may have a higher voltage requirement when first turned ON due to an inrush current, so adding 1 or 2 volts to Vlowest should be safe. A typical value of Vlowest is 9 volts.

For your problem, I made some assumptions:

Vps = 24 VDC
Rcable = 50 ohms
Idevice = 15 mA (assumed average current draw of existing and new devices)
Vlowest = 9 VDC

Vcable = Vps - Vlowest = 24 - 9 = 15 VDC
Icable = 15/50 = 0.3 amps
N = 0.3 amps / 15 mA = 20 devices.

Subtract the number of existing devices connected to the spur and you'll get the number of new positioners you can connect at that spur, less 2 devices (for safety margin).

Hope this helps.

W. Galibut

 

Ther is a PVST system available that will handle your requirements. However, it communicates via Hart rather than FF. But rather than re-inventing the wheel, it will enable you to configure your test procedures, automatically perform the test, and record test results based on smart diagnostics using speed, air consumption and position to predict actuator or valve problems. It can be run on a dedicated PC and not bog down your FF network. Check Metso Automation, Neles ValvGard. Just a thought.

 

The FF technology does not limit how many outputs you put on one network. You can put 8 or more no problem. Some plants have internal practices to use fewer loops per bus in order to limit the impact of a short and to ensure faster response. To limit the impact of a short on the spur you can use junction boxes with current limiters.

To learn more about fieldbus design practives take a look at chapters 3, 8, and 10 of the yellow book "Fieldbuses for Process Control: Engineering, Operation, and Maintenance" buy online: http://www.isa.org/fieldbuses

Jonas