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pressure reducing devices

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spereira

Chemical
Mar 10, 2003
40
Hi, I have a question regarding liquid line pressure reducing devices: we have 3 condensate pumps running, discharging at 8.5 bar for two paralel circuits, one at 8 bar (lowest flowrate) and the other at 1.5 bar (highest flowrate). In order to optmize energy consumtion we're planning on changing the pumps in order to separate both circuits. For the low pressure circuit, a pressure loss of 5 bar is being carried out on the existing level control valves. We'll need to change these valves in order to decrease the pressure drop across them but, keeping one of the existing pumps as spare for both the new ones, if the low pressure pump fails the "old" one will need a pressure reducing device so that the new control valves will still operate in the stroke range defined by the supplier. If we use a restriction orifice (browsing the forum I've found the equations needed for the design) it will be adequate only for the flowrate it was designed for. however flowrate can vary signifficantly depending on the number of drums operating at a given moment. My question being, is a throthling valve the best alternative to the orifice?
Thks,
SPereira
 
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Not sure I completely understand your question but it sounds like you intend to keep one of the "old" pumps as a standby? It sounds like the head provided by the "old" pump will be so high that controlling with the "new" level control valves will be difficult or impractical?

If all of the above is correct the easiest and most flexible way to overcome the problem is to install split range level controls. In this configuration you will have two level control valves arranged in parallel, one covering the low-mid flow range and the other providing the additional capacity to meet the mid-high flow range when required. By splitting the capacity across two control valves you should always have a system that is reasonably controllable, i.e. if the capacity can be accomodated by one of the control valves the other will remain closed.

Hope it helps?
 
SPereira,

If the old pump will be dedicated to the lower pressure system and has a flow range that is adequate but there is too much head, I would look at reducing the impeller size as your pressure reducing device.
 
hi guys, thks for the suggestions.

121202, you've understand it correctly, the problem is that such an high head would make level control impractical (my guess is that it would become some kind of on/off control) so whatever flow rate is needed we need to reduce the "old" pump head since the new valves will be designed for 0.5 bar pressure drop (the low pressure circuit has 4 independent control valves, feeding four different sets of steam drums) - it would be easier to reduce the pump head before splitting the flow through the 4 control valves. I could keep the old control valves in parallel and set up the DCS control routine to use these if the old pump is running but my guess would be that a throttling valve would also do the job...easier.
EGT01, I cannot change the impeller of the old pump 'cos it will also be used as spare for the high pressure circuit.
Regards,
SPereira
 
Why not install a recirc line on the old pump?
Bleed flow back to the suction side. A small bleed would drop the pressure a lot without taking much flow.
Or change the pump speed to resize it?

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