Addition of inerts in MP section of Urea to keep Ammonia in

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  • Author
  • #97308
      • Expert

      Hi all,
      In the shut down condition we can isolate the ammonia booster pump. So we can avoid any effects of leakage on the pump. All other options to increase the pressure of ammonia receiver can only be used at the time of plant start up.
      In this way we can save Nitrogen and steam consumption.

      Nasir Hussain

        • Participant

        Dear Imran,
        First of all, the shutdown action for MP section is to block it from up/ downstream. If necessary even the MP NH3 receiver to maintain the vessel pressure, and it preferable needs to be higher than the saturation pressure equivalent to the operating temperature of the NH3 at the time of shutdown.
        NH3 boil out is caused by two reason, one is the sudden drop in loop pressure at constant temperature [relates with proper isolation of the loop] or secondly the increase in NH3 receiver vessel temperature [relates with continuous operation of either HP NH3 feed pump or the booster pump on recycle].
        During shutdown you might face either of the two devils as follows, and the actions required are different for both.
        Low pressure scenario:
        During low pressure, the NH3 will reach its boiling point even at lower than the operating temperature. The step you need, is to apply LS jacketing steam at recycle line of booster pump, if available [Saipem patent]. This will introduce vapor NH3 to the vessel and sharply increase the pressure of the vessel above or equivalent to the corresponding temp.
        High temperature Scenario:
        During high temp, you can import cold NH3 either from the storage or from backend at relatively lower temperature. This action requires time before the booster pump to be taken in operation for start-up because the pressure will drop when we import cold NH3 to the receiver vessel.
        I may suggest a modification here, as already implemented at FFC-II, provision of cold shots [fresh cold NH3] in to the suction of booser pump. It will drop the temperature thereby stop the boil out of NH3 in pump suction. and will avoid pump seals damage.
        Hope the explanation and the modification will suffice your requirement.

          • Participant

          Dear Imran,
          Whenever urea plant is shut down for longer duration HP loop is isolated from rest of downstream section. If during the shutdown there is a potential of CO2 slippage from MP absorber to ammonia recovery section then Ammonia receiver is also isolated from rest of MP section. To hold the MP loop pressure keep set point of MP loop Pressure controller should be at least 1-2 kg/cm2 above from its PV value. If pressure is further going down then it indicates the passing problem in MP loop pressure control valve then firstly isolate the valve and control the loop pressure via its bypass valve. To make the pressure of MP loop use LS jacketing steam at the common recycle line of ammonia booster pump, this is the only possibility to make up the MP loop pressure during the plant shut down.

          Best Regards
          Mohsin Pasha
          Fauji Fertilizer Company Limited

            • Expert

            Hello Mr. Imran!

            I hope you are enjoying good health.
            First thing is that after shut down, most important action is that you have to hold the pressure of the MP section by MP Vent controlling. And this pressure is because of inert present in it already. So no need of extra inert.

            One more thing that is coming to my mind is that after shut down of plant HP LOOP pressure process variable should be on PV-tracking. So that inert transport to MP section should continue.

            If you are talking about the addition of inert after the shut down urgently then use of nitrogen is not feasible because Mr. RVNESARI utility nitogen pressure is lower than the MP section pressure. It will seriously contaminate the utility nitrogen header that may lead the plant towards equipment damage. So this is not possible in my opinion.

            Muhammad Zeeshan Shoaib
            Fauji Fertilizers Company

            Ramchandra Nesari
              • Participant

              Hello Idris!

              Normally there is provision to add nitrogen in MP section particularly in Ammonia Receiver, which can be isolated from MP Absorber. So before starting Ammonia Booster pump, you can raise Ammonia Receiver pressure by adding nitrogen and then start the pump. The seal leakage is not due to absence of inerts but dropping the temperature of Ammonia in the receiver.

                • Participant

                How can we add Inerts in MP section when plant is shutdown and no inerts are coming from HP section. We have issue of Ammonia boil out in the absence of inerts that cause seal damage in Ammonia booster pump.

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