Pressure Drop in the BFW System - Study more, Save More

Objective
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.

Existing System
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.

Pressure Survey
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

It is evident from the above data that the pressure drop evaluation was matching with the actual plant condition.

Actions Taken
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.

Conclusion

This simple exercise is just to indicate that if you wish you can identify opportunities for energy saving anywhere in the plant. This exercise resulted in a saving of ~0.7 TPH of MP steam used for turbines which was equal to Rs. 40 Lac/Year OR ~100,000$/year.

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pH-phosphate Co-ordinated Graph

During one of the studies of boiler pH vs phosphate concentration I found that the problem of data plotting takes lot of time manually & chances of error are also there. For a clear picture of the system co-ordinated pH-phosphate diagram is must.

Therefore I developed the Excel sheet for the same. In our case we were having 3 boilers B-601, B-605 & B-606. From a standard curve I derived the correlation & put the data in Excel sheet called "Data". Now you can put your pH against phosphate concentration in the corresponding cell. For example say for B-605 you need to put pH on F column as 9.70 against phosphate value of 3.56 in row no 260.

This diagram is useful in identifying the effectiveness of your boiler pH control program. After getting this curve you can have an analysis of Caustic gauging zone, Or Acidic Zone or localized attack / proper system.

Using this sheet we maintained very well pH control system.

So Use this file & improve your boilers health.

To Download the file , just click .... pH-Phosphate Co-ordinated Diagram

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Bonus: Other problems identified

During the toubleshooting of ammonia synthesis loop pressure I found that there was some problem across the gas-gas exchanger bcoz its calculated temperature was not matching with the measured temperature. So we checked the Temp gauge at the first place & found it faulty. It was replaced & again temp mismatch was observed but this time difference was lower.

Therefore it was decided to measure the ammonia concentration in the hot gas at the exit for any leakage confirmation. Upon analysis it was found that the exchanger was leaking. It gave an opportunity prior to a major failure & major downtime to rectify the problem in a planned shutdown. On opening it was found that there was minor leakage from the seal packings.

The above probelm also contributed slightly to higher loop pressure due to higher ammonia concentration at the inlet of reactor compared to other plants.

After rectifying this we again found that actual ammonia at reactor inlet was still high. So another simulation was done. This time ammonia liquid separator was the culprit. The VLE data was indicating much lower ammonia concentration than actual one. So after few data points it was decided to check its demister pad for physical condition. On inspection it was found totally damaged & therefore ammonia carryover was taking place.

The rectification lead to a increase of 6% in the capacity at the same loop pressure.

Thus finally this program proved its value to the management and investment they made by sparing me for a month or so to prepare it. I thank again Mr. Pankaj Shah for giving me this opportunity.

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Ammonia Synthesis loop pressure - Mystery solved

This post onwards, my postings are not in actual order of occurence. The next job I did was the problem solving of higher loop pressure in ammonia plant synthesis loop. The loop pressure was running by 10% higher value i.e. ~220 Ata compared to 200 Ata in other similar size, identical, same technology plants of same vintage and company was unable to identify the reason for last 12 years as it was a problem since commissioning.

When I evaluated this system I used the ammonia sysnthesis converter simulation program developed in C++. The major first difference identified was that the overall conversion of reactants into ammonia was higher than other companies (We compared our data with identical plants). The reason of this difference was identified that the second bed in the converter was giving mor econversion & first bed was giving less compared to others.

Ammonia converter S-200 radial flow type Haldor Topsoe make has two beds with one interbed exchanger. Feed gas first goes to the shell annular space rises up providing cooling to the sheel & then enters to interbed exchanger & is preheated to the desired temp range. Then it goes to first bed radially exits from it at higher temp & goes to interbed exchanger shell side for cooling. Relatively cold gas enters to second bed & exits from the bottom.

Now on identification of above reason of higher overall conversion, less conversion in first bed, higher conversion in second bed was sufficient for identifying the problem area.

On simulation again with the actual conditions, we found that some amount of additional gas is required to enter to second bed directly wihtout passing thru first bed. At this point came the idea of some internal bypass or leakge in the exchanger first which was ruled out as it will result opposite to the actual phenomenon. It will lead to higher conversion in first bed.

So finally upon detailed analysis of reactor internals drawing for few days, the only possible way of gas flow to second bed was identified thry bottom support plate sealing.

Based on this it was decided to open the reactor in next turnaround but it was openend earlier due to basket failure which was also an unique phenomenon in this particular reactor. Such failures are not common but it was happening in my company for 4-5 th time.

Surprisingly it was found on inspection of plate that tightening bolts were loose & they were only hand fitted & probably engineer missed the step. This expalined both the reasons of high loop pressure & repeated failure of converter internal pipe. SO COSTLY..........MISTAKE..........A good learning for all of us.....................

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