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October 16, 2018

How To Benchmark Steam Cost for Your Plant Operation

My name is Kelly Paffel. I’m technical manager for Inveno Engineering, LLC, located in Tampa, Florida. We’re a domestic and international firm, specifically in steam system engineering. Today I want to talk about steam cost, one of the most important benchmarks in the steam system.
Steam cost benchmark. Why benchmark? We need to set goals to reduce the steam cost. Also, for trending for changes in the system. This provides knowledge to make changes if the cost index changes. So if the steam cost increases, we know we have to take corrective action. Accountability. Charging for different production areas for steam usage. If we don’t have a steam cost, then it’s free and everybody will not look at saving energy.
Why benchmark? Knowledge to all plant personnel. Energy saving calculations. Example would be this here. Venting steam to atmosphere. With this here, if we didn’t have a steam cost, then no one would know exactly how much this is costing us. So this has a substantial energy loss, but we need to know what the cost of steam is for the plant.
What affects steam cost? Well, of course, fuel cost. Lower cost of fuels will provide a lower steam cost. Simple as that. How we’re purchasing fuel from the different sources. Operating steam pressure. Lower operating steam pressure will provide a lower steam cost, but caution. Steam pressure should be reviewed to ensure steam pressure is at the optimal pressure to meet the plant process requirements. Also, the boiler has a design operating pressure, and you want to be at that design operating pressure.
What affects steam cost? Percentage of makeup water or percentage of condensate return. Higher percentage of condensate return to the boiler operation will lower the steam cost.
Last, the boiler efficiency. Higher boiler efficiency will provide a lower steam cost. Is the boiler operating at peak performance? Boiler efficiency is calculated using PTC 4.1. Boiler efficiency should be monitored daily.
Unloaded versus loaded steam cost. When establishing a steam cost, it’s important to determine the true cost of steam being used in the steam system. People always say, “What is loaded cost and what is unloaded cost?” Loaded steam cost has all operational costs into it. Utilities, operations, capital purchasing, administration, et cetera. Unloaded cost is just the fuel cost that we’re using for the boiler operation.
Unloaded cost is the basic comparison between how much steam we produce and what the cost of the fuel that we are purchasing. Basic equation for calculating cost of fuel being used by the boiler, difference in steam being generated by the boiler and the feed water, and, of course, boiler efficiency.
So the calculation would be this here. Sc equals af(Hg – Hf) divided by 1000 divided by boiler efficiency. AF is the fuel cost in million BTUs. Hg, the enthalpy of steam in BTU per pound output of the boiler. Hf, enthalpy of the boiler feed water in BTU. That is the water coming in from the deaerator. Nb, true boiler efficiency (ASME PTC 4.1), a percentage. 1,000 is the steam cost we measure in 1,000 pounds per hour.
Here we have a diagram here. This gives us the steam cost. Of course, fuel cost, boiler efficiency, and the water coming in from the deaerator headed to the boiler room and the steam going out of the boiler. So there’s our four factors that we’re going to put into the steam cost.
As an example, we’re going to use a boiler today operating at 240 PSI. This boiler operating at 240 PSI right here and our deaerator is operating at 15 PSI. We’ll say our boiler efficiency is at .79. But there’s always unknown down here is what is the condensate being recovered, and we’ll talk about that in a few slides.
Example. Steam being generated at 240 PSI. Deaerator is operating at 15 PSI. Fuel cost is $4.50 per million BTUs, and the boiler efficiency is at .79.
So our calculation is $4.50 (1201.8), which is the enthalpy of the steam leaving the boiler, and 208, the [inaudible 00:05:42] energy of the condensate or feed water going into the boiler, divided by 1,000 divided by .79. We come up with a fuel cost of $5.60.
Now, there are calculations that we can do. This cost of steam calculator is up at our website free of charge. You can download it, but I just wanted to do a couple of examples here. So we already calculated out and said our boiler efficiency is at 79% and our cost of steam is $5.60, which is located right here.
Now, if I had somebody come in and set combustion on the boiler and increase my boiler efficiency to 84%, and then my steam cost is reduced to $5.27. So just by adjusting the boiler to the peak performance of the boiler, we will reduce our steam cost.
So this is just another example. If we have no condensate return and we use make-up as 70 BTU/lb and go through the equation on the system what we did previous and we just use the 70, and now we had a steam cost of $6.44. Almost a dollar per 1,000 pounds more, because we’re not returning condensate. So we want to be proactive on our condensate recovery, because it does affect the steam cost pretty dramatically.
Plants can have more than one steam cost, depending on the plant operation. This here is a boiler that’s burning methane gas. So we’re using methane gas and we’re also using natural gas in this boiler. So it depends on the mixture of methane gas, which is definitely a lower fuel cost. We can have a different steam cost. Some plants have four different steam costs, depending on where they’re generating their steam.
The other thing is just understanding the boiler feed water. We come up with … We have gravity condensate return. We have low pressure return. We have medium pressure, high pressure, and we also have make-up water. So we devised a calculation here. HF equal to percentage of gravity plus the percentage of low pressure plus the percentage of medium pressure plus the percentage of high pressure plus the percentage of make-up water. Then we can come up with a more true steam cost based on what our system is doing. So we’re taking encompassing the whole system. The previous cost we were doing, we were just using the deaerator, and the deaerator uses steam to bring the temperature of the make-up water and the return condensate up to the operating pressure of the deaerator. So it kind of hides what’s happening out there in the system.
The other thing is that understanding unloaded cost and loaded cost. Loaded cost, I said before, takes in steam generating expenses, as operator, water, chemical, maintenance, administration. All the aspects of the operation. So we calculated out $5.40 per unloaded cost. Loaded cost is usually two to two and a half … 25 times that of unloaded. So it would be almost $10 per 1,000 pounds. Over $10 per 1,000 pounds of steam.
Thank you for listening to our presentation today. If you have any questions, you can call us. Our information is down here below. Our website is www.invenoeng.com. Have a great day.