What is the ‘Hold-Up’ In Considering Pneumatic Hold-Up Time in Your Facility?
What is “Hold-Up Time”? In Pneumatic power systems, “hold up time” is the length of a time a compressed air supply can continue to meet downstream pressure demands after the failure of the upstream compressors – ie: With zero system input. Compressed air is used to store Energy. An air compressor consumes direct energy (typically in our systems via AC power), and converts it to potential energy by means of compression. As the compressed air is released to atmosphere through devices and regulated end uses downstream, that potential energy performs “work” – opening and closing or holding shut valves, conveying media, turning turbines, etc. When downstream demands are critical – when a complete system shutdown can cost not just lost revenue but lost lives, “Plant Hold-Up Time” becomes a paramount issue. A sudden loss of pressure due to system failure must be avoided – or at least delayed until other processes can be brought online or losses can be mitigated. This article addresses another way to do just that.
If your employed in the Oil and Gas Sector and are involved in compressed air systems, then you have undoubtedly seen the gigantic compressed air storage tanks at sites where Plant pneumatics are integral components of safety systems. You may have wondered how those towering vessels were sized and may even have wondered what they might have cost. What you likely haven’t considered is whether there was a better way to store that volume of air. A less costly way.
Average Instrument Air System designs demand 20 minutes worth of compressed air storage at base load plant demands, be available in the event of a complete system failure. In the case of system sized for 100 cfm average outlet flow at -40C PDP at average 120 psig skid edge pressure, this would mean we would need to have stored 20 minutes x 100 cfm = 2000 cubic feet (14,960 US Gallons) of compressed air at pressure. But at what pressure?
Simply because the system was designed to deliver 120 psig at skid edge during normal operation doesn’t mean that the critical end uses need 120 psig to operate. The engineers who designed that system left safety factors in the skid edge pressure requirement so that end devices wouldn’t perform poorly due to pressure drops in the compressed air distribution network, or due to poor system maintenance. While minimum operating pressures will vary both by device and by site, the average minimum pressure calculated for will be in the 80 psig range. This means, if the system when working as intended can deliver 120 psig at skid edge in the flow and purity required, our plant hold up time vessel will be storing a differential pressure of 120 psig – 80 psig = 40 psid.
This differential pressure – the difference between the storage pressure at the start of the failure event and the minimum pressure required to meet the critical needs downstream – is used to reduce the vessel size from the 2000 cubic feet noted above down to a fraction of that size; in this case 369 cubic feet of storage – or 2,760 US Gallons. Now that’s a much smaller tank; but still a bigger vessel than can be easily fit into a site building. As a result, we commonly see large vessels installed outdoors on sites – custom engineered vessels designed to handle wind loads and cold temperatures; features that drive up their costs. They require special foundation work and paint protection against the elements. Given these tanks are sized to be used only in the unlikely event of a full system failure, there be a more cost effective way to store this volume of air?
The answer is “Yes”, and it’s in the differential.
Look: If we were consistently delivering only the minimum required plant pressure at skid edge (80 psig in this example), then to have 20 minutes of hold up time for a 100 scfm downstream demand, we would need a 14,960 USG vessel. 80 psig – 80 psig = 0 psid.
By increasing normal skid edge pressure to 120 psig, we have reduced the required tank size for that same 20 minutes of 100 scfm demand at 80 psig to a mere 2,760 US Gallons. 120 psig – 80 psig = 40 psid.
Now imagine instead of storing the hold up time air at 120 psig, we stored it at 500 psig. 500 psig – 80 psig = 420 psid. Now the required hold up time vessel is only 35.14 cubic feet in size; or 262.8 US Gallons. That’s a reasonably small tank; easily fit on site in any building, or added directly to the Instrument or Plant air skid. But can we economically achieve this higher pressure?
While sizing the system to consistently deliver a higher skid edge pressure merely to allow for a smaller hold-up time tank would be both costly and grossly inefficient, the infrequency of the need for “hold up time” storage means we don’t need to fill such a vessel in seconds or minutes. That storage is used only in the case of plant wide emergency. It would be wise to have it ready at commissioning, but if it takes a number of hours to fill that vessel, that is inconsequential to the process. Since the tank itself is not technically intended for “dry storage” (although it is often used as that), it can be a supplemental tank on the system – fed by the standard air compressors but through a much smaller BOOSTER compressor.
Booster compressors take the system inlet air at pressure (in this case, 120 psig), and re-pressurize it to allow for higher pressure storage (in this case, 500 psig). That stored high pressure air can then be isolated from the downstream supply lines via a simple normally open solenoid valve (which opens on system failure) and a precision regulator – regulating it’s outlet flow at a stable (minimum) 80 psig in the event the system crashes. By adding an additional safety relief valve past this regulator in the system, we ensure high pressure air can’t bypass the regulator and cause problems downstream. With the hold up time vessel pressure contained, the booster shuts down and will not start again until and unless an event occurs that demands the use of the stored pressure.
CAE hasn’t just theorized this system design, we have put it into practice. By installing a small high pressure receiver with a booster compressor and the safeties noted above, we were able to save our client $72,340.00 in capital costs – not including the cost avoidance realized from not having to have a custom vessel shipped separately to site (estimated at $1,280.00), erected on a newly poured engineered concrete pad (est $9,100.00), and tied into the system in the field (est an additional $3,460.00 in labor and materials). A total savings of $86,180.00 on a package that cost just under $200,000.00 – a whopping 43% savings.
When it comes to pneumatic power – differential pressure makes all the difference in the world. Once you understand that, you understand exactly what you need to do to build a Compressed Air System.
Reimund (Ray) Krohn is Central Air Equipment’s Product Specialist for Engineered Projects. He has been working in the compressed air industry for 22 years and is a US Department of Energy certified AirMaster+.