Yesterday the only 1200 kW Steam Turbine in my mill broke down. The repairs will take between 30 and 45 days depending on how fast we can get the spare parts. I am under pressure from my boss to keep the mill running. My questions to members are:

Without sparing any cost on fuel, can I run the mill with power from the Diesel Generating Set only and Steam from the Back Pressure Receiver (BPR) using the by-pass line from the main steam line to the BPR?".

Will I be able to achieve my usual 45 tons FFB per hour?

Please anybody ... Help...

To run the mill at your rated capacity (or usual capacity) without running the turbine and with 100% process steam from the bypass line from the main steam header to the BPR, you will need to operate the boiler at your usual working pressure. If you run the boiler at, say 100 psig, you can throttle the steam pressure down to 45 psig but the flow rate of the steam will be very much lower. This is due to the difference in specific volume of steam at 100 psi (0.273 m3/kg) and 300 psi (01.100 m3/kg). Hence for the same pipe size, higher boiler operating pressure will deliver more steam to the BPR.

The next bottleneck to getting enough quantity of steam for the milling processes is the sizes and length of the Main Steam pipe and the Bypass pipe. A 6 inches diameter steam pipe can deliver up to 16,193 kg per hour of steam at 10 barg. The same pipe can deliver about 30,250 kg per hour of steam @ 20 barg. In both cases, steam velocity taken at 40 m per sec, assuming saturated steam quality. For superheated steam, the velocity can be higher, up to 70 m per sec. It should be noted that high steam velocity in pipes will produce noise and erosion of the pipe wall.

Say your normal milling capacity is 45 tons FFB per hour. For this you will need about 22.5 tons of steam @ 45 psig per hour. If you run the boiler at 10 barg, assuming 6 inches diameter steam pipe, you will only get about 16 tons of steam per hour. However if you run the boiler at 20 barg, you will get about 30 tons of steam per hour. Of course the final flow rate will depend on the length of main steam pipe, the number of bends and the pressure drop (caused by the velocity of steam).

In most mill, the Bypass Steam Line from the Main Steam pipe to the BPR is only designed for make-up steam, not full flow of the required process steam. The PRV and the Bypass line at the PRV are usually smaller in diameter and may further restrict the flow of steam. So you will need to adjust the boiler operating pressure to get the optimal steam flow from your system.

It should be noted that the steam in the BPR will be superheated after the reduction in pressure from the Main Steam pipe into the BPR. It may not be ideal for sterilization, but this is a compromise you will have to make to get the mill running until your turbine is repaired.

Hope this will give you some idea to get the mill running as close to your rated capacity.

Thank you all for your feedback and ideas.

Just like Wesley, I can only process at about 50% of my usual capacity with the boiler operating at 100 psig.

Is it possible to process up to my rated capacity by using steam from the boiler through the bypass line into the BPR?

No..the capacity was only about half of the rated capacity.

This is an interesting topic. We had a similar situation before though the downtime was much shorter. Our boiler has superheater. We lowered the operating boiler pressure to 60 psig and bypass the Steam to BPR. But I was very worried if there will be any damage to the superheater later due to probably overheating?

Hi Suria, thanks for your feedback. You are concern about if there is adequate quantity of steam flow through by pass line for milling operation. My suggestions are as follow; 1) please check your by pass pipe size against its flow rate, you probably required maximum of 45 * 600 kg/hr for 45 ton FFB throughput. Anyway, try to do it at 45*500 kg/hr. 2) at comparatively higher boiler pressure will expand into comparatively higher volume of steam at back pressure receiver. Thanks.

Hi, Suria, Milling with Electrical Power from Diesel Gen-set. Your question short of some important details, please allow me to make the following assumptions. 1) you are not able to lease a unit of steam turbine during this period of time. 2) combined capacity of your diesel gen-set is approximately 1,000 kw and your electrical power consumption during milling is approximately @ 16 kw/ton FFB or even slightly lower. This will give you total electrical power requirement of 720 kw @ 45 ton/hr throughput. If so, you can utilise your diesel gen-set for milling operation. Most Diesel engine cooling system does not allow it to take 100 % load, maximum is 80 %. This coupled with the fact that milling operation could be at least 16 hours put further stress on Diesel engine cooling system. Sorry for late reply. Thanks.