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Minimum Flow System




My friend, In this posting we will discuss about the pump recycle systems. The pump shall be protected from minimum flow, due to the following reasons;
1. Thermal consideration which has effect to the pump efficiency (The efficiency will decrease significantly at low flow)
2. At low flow, Internal recirculation will be occurred
3. Increase load impeller (both axial and radial)
4. liquid containing a large amount of abrasive particles must flow continuously through the pump. at low flow, the particles can circulate inside the pump and erode the impeller.

The three types of pump recycle systems which are generally applied are ; 
1. Continuous recycle system --> use Orifice
2. Control loop system and --> use Control Valve
3. Automatic recycle system. --> use ARC
My friend, The followings are short guidance to apply those types.

1. ORIFICE
  •  This system will provide continuous recycle flow whenever pump is running, regardless of the process demand.
  • Typically applied for pump below 50 hp (38kW), or small capacity.
  • Don’t forget, you must consider the additional capacity of the pump (the capacity is included recycle flow)
  • For initial estimation, 30% of rated flow can be used as recycle flow basis.
  • Take care for high pressure drop condition, is there any flashing? Two phase? Or vibration? If two phases is occurred, the orifice should be located at the end of the recycle line.


2. CONTROL VALVE
  • When the minimum flow is reached, the valve start to open. The flowmeter shall be located at pump discharge side (installing at suction side will decrease the suction pressure, decrease NPSHa )
  • The flow meter should be located before(upstream the) recycle line connection.
  • Be careful with high pressure drop, control valve may be noise and vibration (an orifice in series with the control valve may be required)p


3. AUTOMATIC RECIRCULATION 
  • This type is the spring loaded, combined recycle and check valve
  • The spring loaded is set at a pressure correspond to minimum flow (based on pump curve) where the recycle starts to open.
  • Check this link for the detail how the ARC valve work.
  • No need additional check valve in the discharge system. The ARC provides back flow protection.
  • Take care with pump with high discharge pressure ( while low pressure source) . The orifice may be required at the recycle line to avoid very big change of momentum

General Guide.
  • Recycle line should be routed back to the source (tank or vessel) NOT to pump suctionBubble of two phases can be formed during pressure reduction by orifice or control valve. It may be introduced into the pump. 
  • Routing to suction line may causes excessive temperature build up in a very short time.
  • For 2 x 100% operation, the recycle line can be connected at combined line.
  • For 2 x 50 % operation, each pump shall have its own recycle system.



-          

     Generally, the recycle system is not shown in the PFD. There are two specific pumps when a recycle system should be included in the PFD. These are the BFW pump and pump with the high head or capacity. (discharge pressure > 1000 psig)


That's all I can share this time.
Hopefully this is useful for you..

Wait ..wait..I have a question for you.
Here my simple question.
Let say, 100 gpm liquid shall be transferred via pump to a tank. The recycle is continuous of 30 gpm use orifice.

How you state the pump capacity in the process datasheet?
A. Normal = 100 gpm , Rated = 100 +30 = 130 gpm
B. Normal = 130 gpm, Rated = 130 x 1.1 = 143 gpm
C. Normal = 100 gpm, Rated = 1.1 x 100 + 30 = 140 gpm

hhmm,,,can you explain WHY ?  it is simple, isn't ? :D

Thank my friend..

28 comments:

  1. Is it option A?

    ReplyDelete
  2. Is it B Sir?
    I think it is becoz the recycle is continuous, so the normal flow is 100 + 30 = 130 gpm,,
    Then use 10% for the margin,,

    Please ur correction Sir?

    and thanks alot for the nice posting,, ^_^,,

    ReplyDelete
  3. it is B.

    The recycle system is out of vendor pump battery limit, so the pump vendor will not consider whether the recycle system use control valve or orifice or ARC.

    We can specify normal flow = 100 gpm + minimum flow in the datasheet.
    in case the minimum flow has not been known yet.
    therefore vendor will advise the amount of the minimum flow.

    please note : required minimum flow will not always to be 30% normal flow.

    ReplyDelete
  4. Guess what its B. it will be always advised by the vendor, sometimes we need to see the correct data or manual from them. Nice Posting, really educational.

    ReplyDelete
  5. Hi Triyanto SR,

    First of all I should say..."Thanks" for sharing this Post very nicely...

    I would be very grateful to you If you could explain the answer of the following questions which arised while I was going through this post...

    Q-1. If we consider Option-1 (Continuous Recycle System) and there will be a two phase flow outlet of the orifice then what kind of design options we can implement?

    Q-2. What is the difference between rated flow and Operating flow? Why we are not considering the minimum flow based on maximum flow-rate of pump?

    Q-3. What are the prons and cons If the orifice is installed in upstream or downstream of the control valve in Option-2 (Control Loop System using Control Valve)?

    Q-4. You have mentioned in General Guide that If a minimum flow line is routed to pump suction then the liquid temperature can build up...How? As we have pressure drop from high to low pressure then in that case the temperature should drop to the suction temperature...?
    In refinery, the pump which handles petrol or methanol always has minimum recycle arrangement in order to bring down the discharge temperature for safety reasons...is it right?

    Q-5. In general guide you have mentioned the two cases which is based on operational criteria...
    (1) For 2 x 100% operation, the recycle line can be connected at combined line...Do you mean that two pumps are working at the same time with 100% capacity....or one pump is running at 100% capacity while the other is standby pump...?
    (2) For 2 x 50 % operation, each pump shall have its own recycle system....?

    I know it will require a lot of time for you to answer the following questions but I would appreciate it if you could answer....some of the questions will look very silly to you but to me its very important to know...I am sorry for that in advance...

    Thank you once again...

    ReplyDelete
  6. May I try to answer the Q-4 based on my common senses? Perhaps the temperature will be rise because the pump continuously adding its shaft work on low flowrate. Not all of the work absorbed by the system as work. Some of them absorbed as heat due to things like friction. We know from basic physic for heat it will be Q=F.Cp.delT. Because the F is low, consequently the temperature will be rise (because the rise of delT) and there will be a built up of temperature.
    Please correct me if I'm wrong. :)

    ReplyDelete
  7. Mas Tri

    If I may added, the reason of using minimum flow is also for safety reason, in related to the reason no.1, aside of effect to pump efficiency.

    As you stated in point no. 1

    Quote:
    "Thermal consideration which has effect to the pump efficiency (The efficiency will decrease significantly at low flow)"
    Unquote

    In case of block discharge, the fluid temperature will increase. If the fluid is flashing fluid e.g. LPG, the fluid may get flashed. It will cause pump cavitation and result in impeller damage. In addition, mechanical seal will be broken due to vapor formed, and result in hydrocarbon leak to atmosphere, which lead to potential fire if ignited.

    ReplyDelete
    Replies
    1. I think it's more about the damage of the pump driver (the motor) brought by the increase in pumping energy just to overcome the blocked discharge line. Cavitation in the first place will occur at low suction head causing the process fluid to flash or vaporize resulting to a damage to the impeller.

      Delete
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