9 replies to this topic

[PENGG]

1. What are the criteria to decide whether to provide orifice or control valve in minimu recirculation line ?
2. What are the merits and demerits of orifice and control valve ?

Edited by PENGG, 02 January 2013 - 08:25 AM.

[mech-che]

1) suction energy loss/pressure drop
2) cost

[S.AHMAD]

1. What are the criteria to decide whether to provide orifice or control valve in minimum recirculation line ?
You need to determine the breakeven point. Past study showed that the breakeven point is around 50 HP. If the pump is smaller than 50 HP, orifice is cheaper long term cost otherwise better to install control valve.

2. What are the merits and demerits of orifice and control valve ?
Orifice plate is cheaper investment cost but need extra energy for pumping since we need to add the minimum recirculation rate to the design rate. Control valve is more expensive investment but there is energy saving. Therefore there is an optimum (breakeven) point that you need to determine.

Edited by S.AHMAD, 04 January 2013 - 03:21 AM.

[GAURANG PATEL]

1. What are the criteria to decide whether to provide orifice or control valve in minimum recirculation line ?
You need to determine the breakeven point. Past study showed that the breakeven point is around 50 HP. If the pump is smaller than 50 HP, orifice is cheaper long term cost otherwise better to install control valve.

2. What are the merits and demerits of orifice and control valve ?
Orifice plate is cheaper investment cost but need extra energy for pumping since we need to add the minimum recirculation rate to the design rate. Control valve is more expensive investment but there is energy saving. Therefore there is an optimum (breakeven) point that you need to determine.

Dear S Ahmadji,
With reference to your point no 1 answer that for below 50 HP motor pump orifice is better and for more than 50 HP motor pump control valve is better, I would say to relook in that cut off. Reason is as below.

Lets consider 33 HP motor pump that is equal to around approx. 24 kW motor.
Lets consider minimum recirculation is 30 percent of rated flow (It may be even higher up to 50 percent but lets take least value).
At that 30 percent flow hydraulic power requirement will be also 30 percent of original hydraulic power (Hydraulic power for rated flow).
Practically pump and motor efficiency will be less at less flow rate but still if we consider it is constant than Motor power consumption at min recirculation flow will be 30 percent of original motor power for rated flow.
That means at min. recirculation flow motor will consume 7.2 kW power.
For 24 hours continous operation it will be 173 kWH power consumption.
Now 1 kWH power consumption costs 2 Rs. (This value is for India. In some company that value is even more than 2 Rs.)
That means cost of running motor for one day at 30 percent min. flow is 345 Rs/Day.
Now if we install orifice as per your mentioned criteria than we are totally wasting 345 Rs/Day.
Now lets consider if we install control valve. Control valve (FE/FT/FV/construction/maintenance) may costs starting from 1.7 Lacs Rs to as high as 15 Lacs Rs (This values again applicable for India). Lets take 7 Lacs Rs cost of control valve.
In that case pay back period for control valve will be 5.5 year. That is reasonably good attractive payback period.
So as per that even for 33 HP motor pump control valve is better option.
Remember in above calculation majority variables are selected conservatively still pay back period is five years. It would be even less with exact actual values.

[kkala]

Post no 4 contains interesting economic thoughts. I understand that 1 Lac = 100 000; maintenance cost had better shift to operating cost (of 345 Rs/d), but this may be minor. Besides min flow control valve will actually have some flow, in case discharge flow "demand" gets lower than (assumed) 30%.
I have not seen orifice on the minimum flow line here, probably because centrifugal pumps in refineries are rather big.
Orifice is judged to have a safety advantage over control valve, of course in case of clean liquids (without solids). A mechanical min flow control valve (e.g. yarway type or equivalent) is more reliable than an air actuated control valve (being fail open); that's why the former is installed in critical services (liq NH3, BFW pumps). But I think even a yarway type valve can fail to open in rear cases, while the orifice is always open.

Edited by kkala, 10 January 2013 - 04:16 AM.

[S.AHMAD]

G Patel
Congratulations. You have done what a process engineer should do as part of their job as a process designer. I suggest that you also look at other minimum flow e.g. 10%. 20%, 50%, 70% etc. so that you may establish the breakeven point for India. You may also do sensitivity analysis such as for 10% increases of electricity cost etc.
That 50 HP guideline was done by a company in US. I have no idea the basis of the study. What important now is that you understand the method on how to select between orifice and control valve for minimum recirculation mechanism. The 50 HP quoted as an example.

Edited by S.AHMAD, 09 January 2013 - 07:42 PM.

[fallah]

So as per that even for 33 HP motor pump control valve is better option.

Hi,

As far as i know, in most cases up to 15 kW installed power it would be more efficient to have a permanent recirculation line fitted with a restriction orifice.

Edited by fallah, 10 January 2013 - 01:37 AM.

[kkala]

It is also pointed out that criteria for acceptable payout time can differ from place to place (depending on financial conditions) even from company to company in same place (depending on policies). In a local fertilizer industry (~1978) acceptable payout time for modifications in the units (realized by Operations) was just 2 years (or shorter). This would be different in India, or in Greece now.
Moreover comparison of two alternatives (one has to be applied) can have a base different to an additional investment in the plant.
For a new project, upper "limit" of installing orifice (control valve for higher than it), or any relevant guideline on what to install for minimum flow, would be clarified in the "design criteria" issued at the start.

Edited by kkala, 10 January 2013 - 04:56 AM.

[S.AHMAD]

1. Good comments by kkala and fallah
2. The results whether 15 kW or 33 kW depends on the basis for the economic evaluation. Some company uses 10% return as the hurdle rate for IRR method.

[Sathis]

what is break even point? and how to determine that