When we consider the cost of steam, we typically consider the cost of utilities such as fuel, water, and water treatment chemicals. We then determine that, in relative terms, steam is not that expensive. As a result, we often see plumes of steam escaping from vent lines and exhaust valves because steam is almost free, right?
But what is the true cost of steam? Yes, we should consider the cost of utilities, but we should also consider the impact that a poorly designed or maintained steam system may have on production quality, throughput or yield. When the steam system impacts a plants’ ability to maximize production, the cost of steam has a major impact on profitability.
We might also consider the impact that the steam system may have on equipment reliability, maintainability; safety; environmental stewardship and regulatory compliance, for the same reasons.
Any or all of these result in significant additional cost to the business that may be directly caused by or attributed to performance issues from steam system.
How can we avoid or mitigate these additional costs and potential risks to the business and ensure that we maximize the performance of the steam system? There are 5 things that you should consider and be on the lookout for.
1. Cold Steam Traps
Cold steam traps are failed closed or plugged steam traps and they can have serious consequences to the steam system and all associated equipment. Just because they are not wasting steam doesn’t mean you shouldn’t invest in repairing them. A healthy steam trap population allows condensate to be removed from the steam system effectively which means that process efficiency can be optimized, equipment is protected and the condensate can be re-used.
An unhealthy steam trap population with bad actors can result in corrosion, waterhammer, wet steam, and freezing.
2. Condensate Systems
As described previously, waterhammer can result in catastrophic system failure where two-phase flow of steam and condensate exists and condensate is allowed to accumulate. In steam systems, this is related to inadequate drainage points or failed closed steam traps. However, because condensate flashes into steam as it passes through a steam trap, there will also be two-phase flow in condensate systems.
What is not always known is just how much flash steam will be present in the condensate system even when all of the steam traps are functioning properly. For example, condensate at 150 psi discharging into a 50 psi condensate system will produce 7.8% flash steam by mass. Not too much. However, when we consider that steam at 50 psi occupies a volume of 6.7 ft³/lb and the condensate only occupies 0.017 ft³/lb, then that 7.8% mass actually represents 97.1% of the volume discharging from the steam trap. It is almost all steam by volume and therefore, the condensate line is actually a lower pressure steam line. However, we don’t install steam traps on a condensate line! It is effectively a steam line without any drainage points and any upward change in elevation, such as an expansion loop or a rise to a higher level in the pipe rack, will result in condensate accumulating at the base of the riser. As this water accumulates and backs up, both differential and thermal shock can occur, which can potentially result in pipe failure and the associated consequences of personnel safety, system shutdown, and repair costs.
We strongly urge anybody that hears waterhammer in the steam and condensate system to take action in determining and correcting the root cause. Spirax Sarco offers our customers on-site technical support to evaluate waterhammer and its root cause in order to make recommendations on mitigation.
3. Steam and Exhaust Vents
I think we all agreed that the cost of steam is relatively low. However, there are situations where, if you waste enough of it, the sheer volume being wasted adds up to a pretty large sum! There are times when during cold weather or at maximum plant capacity, we find we don’t have enough steam and we need to start load shedding. This is a direct impact to productivity and I have known large plants that experienced a steam shortage but elected to pay to install additional new steam boilers rather than repair the leaks or recover the steam from the vents!
One customer, produced an excess of 50 psi steam from waste heat boilers and chose to vent the excess steam to atmosphere through a back-pressure regulator to prevent the steam pressure from increasing in the boilers. And yet, a nearby large heat exchanger was being supplied with higher pressure plant steam when it was consistently operating at a steam pressure of 40 psi. A simple pipe modification allowed for this previously vented low pressure steam to be fully utilized with an automatic plant steam make-up pressure regulator, saving the plant over $2 million per year! Who said that steam doesn’t cost much?
Increasing heat transfer area on heat exchangers and reboilers operating at lower temperatures, along with the use of condensate removal systems designed to remove condensate from the lower steam pressure operations, can be a way to utilize otherwise vented steam and reduce overall plant steam consumption.
Use of Spirax Sarco Thermocompresssors may be considered to recover and reuse low pressure exhaust steam at slightly higher pressure and temperature applications. Spirax Sarco offers our customers on-site technical support to evaluate repurposing exhaust steam streams to determine whether viable opportunities exist and to create sustainable solutions.
4. Condensate Drainage From Heat Exchangers and Reboilers
Whether the heat exchanger or reboiler is oversized or operating at a duty below maximum, it is common to find that the steam pressure downstream of the steam inlet control valve, is approaching or below the condensate system backpressure. When this occurs, condensate backs up into the heat exchanger and then cools. This results in a loss of output, causing the control valve to open to create the higher steam pressure necessary to evacuate the condensate. Then this higher pressure, higher temperature steam increases the heat exchanger output, so that the control valve must now throttle. Now flooding begins anew and the cycle repeats. This results in temperature swings, corrosion, waterhammer, or the potential to run in a continuous partly flooded state. It’s a common occurrence for operators to open a drain valve sending condensate to the floor, or to the process sewer. This increases safety hazards, and reliability risks, and effluent costs. In the event of a tube bundle failure, there is the possibility for hydrocarbon gas to leak into the operating unit. If condensate is drained to the floor, there is a risk of standing hot water pools, algae, or ice formation, and these conditions provide the possibility for slips and in worst cases, burns. If the condensate is discharged to the sewer, it can result in premature sewer failure, due to the temperature which provides a unique set of operational and compliance concerns. Wasting condensate also results in both energy and utility costs, as each gallon lost requires an additional gallon of incoming cold water that requires chemical treatment and heating. Too much hot condensate to the sewer can impact the effluent treatment biology and poses a risk to environmental compliance.
Spirax Sarco manufactures condensate drainage equipment that effectively removes condensate from heat exchangers at all operating steam pressures down to vacuum conditions and safely returns the condensate to the return system.
5. Steam Assisted Flares
Steam assisted flares utilize steam to maintain flame shape; prevent internal combustion; assist air intake; provide turbulence for improved combustion; reduce smoke; provide coolant for fuel nozzles and prevent freezing. It is imperative that the steam supplied is dry and free from condensate. The presence of wet steam, or slugs of condensate, can have a number of effects such as nozzle erosion, steam flowmeter inaccuracy impacting the steam to hydrocarbon ratio, and the possibility for flame-out conditions. In severe cases, slugs of condensate can damage or destroy the flare head. Flare smoking or flame-out conditions result in Tier 1 compliance deviations, and the emission of uncombusted hydrocarbons during a flaring event gives rise to health and safety risk. The formation of condensate slugs is common where the steam piping has not been designed or installed correctly, or where steam traps have either failed closed or are unable to drain condensate during standby conditions.
Spirax Sarco can assess the steam systems to your flares and provide an evaluation and recommendations to mitigate any risk associated with wet steam or the potential for condensate slugs.
 “What Caused Tower Malfunctions in the Last 50 Years” by Henry Z. Kister – Fluor Daniel
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