The objective of this study was to carry out a pressure drop survey in the BFW system to identify the areas where we can reduce the pressure drop, which will help in reducing the BFW pump discharge pressure so as to conserve MP steam in the turbines.
There are two turbine driven pumps & one motor driven BFW pump in ammonia plant out of which motor driven pump remains standby on AUTO mode for an emergency use. Each pump is having a normal capacity of ~200 m3/hr. The steam turbine drives for both the pumps are of condensing types and use MP steam.
These BFW pumps take suction (approx. flow ~295 m3/hr) from ammonia plant de-aerator at ~120°C & ~1.5 kg/cm2g. The discharge of these pumps at ~128.3 kg/cm2g pressure goes to the first set of BFW pre-heaters E-307A/B where it is preheated to a temperature of 165°C by utilizing the waste heat from process gas.
The preheated BFW at the exit of E-307 A/B goes to the next series of heat exchangers E-211A/B for further preheating by the process gas. The provision of a bypass through a temperature control valve TV-84 is made for controlling the final temperature of BFW after both sets of pre-heaters i.e. after E-307 A/B & E-211 A/B. The temperature achieved after E-211 A/B is ~176°C at present load.
The BFW preheated in these two sets of pre-heaters is now divided in two parts. One part of ~130 m3/hr goes to the synthesis loop BFW pre-heater E-502 where, it is preheated to ~273°C temperature by using the waste heat of reactor effluent. After preheating of this BFW, part of it (~65 m3/hr) is used in the adjacent loop boiler E-501 for generating high-pressure steam & balance (~65 m3/hr) is returned back to the front end.
The remaining part of main BFW stream goes to another set of pre-heater E-210 A/B in the process gas circuit, where it is preheated to a temperature of ~ 274°C. The exit of E-210 A/B is then mixed with the return stream from synthesis loop. The combined BFW stream is passed to the front-end boiler drum B-201 through a flow control valve FV-42.
The flow controller FV-42 is also utilized for controlling the level of boiler drum B-201.
We collected the plant data for BFW flow, pressures at different locations in the BFW circuit, temperatures. The Isometric data were utilized for evaluating the calculated pressure drop in the system & was compared with the actual one in the plant at existing load.
The results are as below
Based on the above study, it was found that the excessive pressure drop of 8.9 kg/cm2a was there in the flow control valve FV-42 (present valve opening was ~70%) in the front end. So, it was decided to reduce the speed of the BFW pumps in such a way that the valve opening remains at around 85% which is necessary for a better operability & control on the BFW flow & boiler drum level. In this condition the pressure drop across the control valve was ~4.0 kg/cm2a with a pump discharge pressure of 123.5 kg/cm2g against the present value of 128.3 kg/cm2g.
Further reduction in the speed or consequently, in the pump discharge pressure was not justified due to improper control of boiler drum levels on account of any process variation upstream or downstream in the system.