SGT 700 has a simple arrangement for ventilation of the GTG enclosure, a fan sucks in air from the atmosphere through a filter, pushes into the enclosure from one end and the air leaves the enclosure on other end.

The Dp.filter has no tripping setting but only shows an alarm after the Dp increases beyond a certain value.
The Dp.fan has a tripping on value falling below a certain value thus indication the flow is insufficient for ventilation.

The issue is, when the filters get choked the Dp.filter goes up (as obvious), but the Dp.fan also rises which in theory would be indicative of more flow but it is actually the opposite of the actual condition (since filter as choked, less flow would go through).

The possible explanation I can think of is that with filters getting choked less air reaches the fan suction (Lp leg of Dp.fan) hence the Differential increases.

Imagine a situation where no air flows through the filters, where would the system end up at?

 

We have yet to reply, as it seems you have answered your own point - or have you ?
Now I have logged on and see your diagram, your statement:
The Dp.fan has a tripping on value falling below a certain value thus indication the flow is insufficient for ventilation.

appears incorrect. And measuring DP across the fan is seemingly irrelevent.
On the configuration you have [as opposed to the fan blowing through the filter], a partly blocked filter will increase the partial vacuum between fan and filter. The fan speed would increase to compensate, but so would the DP across it.

On a blocked filter you would have no flow and a vacuum equating to the capabilities of the fan.
The GTG enclosure outlet would be at atmospheric - same as the ingoing side. So you have 2 DP measures, both measuring the vacuum you have but with opposite polarities.

 

First of all, thank you for the reply.
Let me get to it point by point

1. The Dp.fan has a tripping on value falling below a certain value thus indication the flow is insufficient for ventilation.

This is how the system is, current value of Dp.fan is 0.5kpa, if it falls below 0.1kpa machine trips

2. A partly blocked filter will increase the partial vacuum between fan and filter. Yes I agree.

3. The fan doesn't speed up with the filters getting blocked. As I've seen, I think there is no control loop on the fan to maintain a certain Dp.fan

4. You've explained the no flow situation very well.

The issue would be the enclosure would not be ventilated sufficiently (at all) but the dp.fan would still be above tripping? (Static pressure the fan develops is 0.12kpa)

 

It was assumed the fan was either powered by single or 3 phase AC. On reduction of air flow it would speed up slightly to 'synchronous' speed.

If I've provided a good explanation then your first point must be wrong. The fan will never trip due to increase in DP.
I'm mystified why your trip is not across the filter which is more convential, and it matters not what the fan is doing.

 

Yes, i think these two points close our discussion
"1. The fan will never trip due to increase in DP.
2.Why your trip is not across the filter".

I've written to Siemens about it.
Thank You so much once again.