Data errors and uncertainty undermine the accuracy of design calculations. Additionally, the correlations into which we plug these data have assumptions, simplifications and tolerances. So, to ensure the equipment or plant works, engineers often include an over-design margin. This always adds some cost — and can introduce operating problems as well.
Over-design can accumulate through multiple stages. Here’s an example:
1. The process designer adds 25% extra duty to a heat exchanger service to account for uncertainty in fouling rates but asks for a fixed pressure drop in the exchanger.
2. The equipment designer takes this and then adds 25% extra surface area to account for uncertainty in two-phase flow calculations in the service but keeps the same pressure drop limits. The bigger exchanger now has lower fluid velocities.
3. The reliability team reviews the design and correctly identifies the exchanger as highly susceptible to fouling. So, it mandates having two exchangers in parallel with valves to allow cleaning one exchanger without shutting the unit down.
4. When the plant starts up, the performance of the single exchanger doesn’t meet requirements because it’s 56% over-sized (1.25 ×1.25) and fouls too rapidly. Instead of living with the number of cleanings required, the plant decides to place both exchangers in service. The fouling rate goes up again.
The plant calls in a troubleshooter. That person’s first impression on looking at the exchanger is “My, that’s big for this much duty.” Checking shows the tube-side velocity is 1 ft/sec. Yet, experience in this service demonstrates that 5–7 ft/sec are required to avoid extremely rapid and massive fouling.
The solution is to either replace the exchanger or to re-tube a bundle to increase velocities and to only run one exchanger at a time. Both approaches pose problems — not the least is convincing the plant that the problem is the exchanger is too big, not too small.
Different types of equipment when over-designed create problems in disparate ways. Understanding how equipment fails is important to knowing how to avoid the failure or how to fix it.
But first, correctly control over-design. Unless there’s an overwhelming case for over-design for multiple purposes, only add margin one time. Clearly state where to add it and explain which parameters must be kept and which can be allowed to slip. In addition, explicitly ban adding margin anywhere else. This helps avoid having to call the troubleshooter in the first place.
For centrifugal pumps, don’t simultaneously request over-design for discharge head and capacity. Due to the flow characteristics, adding margin for both will yield a pump susceptible to inlet recirculation. This problem looks a lot like cavitation from insufficient net positive suction head but is different. Specify over-design for either a flow rate or a discharge head, not both.
For heat exchangers, if accommodating fouling requires over-design, specify a minimum velocity and allow some extra margin for a higher pressure drop for the over-designed exchanger. The objective is to add surface area but not reduce fluid velocity. For shell-and-tube exchangers, the best approach is to add length, not diameter; this will result in higher pressure drops compared to an exchanger without the margin.
For new distillation towers, vendor correlations include about 15% margin on the vapor handling rates and up to 100% margin on liquid handling rates. Adding extra margins tends to increase costs and create problems in reducing effective turndown at handling low rates. These problems can become serious in services with high vapor volumes and low liquid volumes — and can lead to significantly degraded performance, particularly when using trays. I recommend specifying the rates and turndowns you need, and letting the internals vendor deal with the detailed design.
Over-sizing of fired heaters also creates problems. Most larger heaters have difficulty with flame control when operating duties drop below 50% of design rates. Keep track of turndown requirements and avoid making the heater too big.
For control valves, too large a valve can make control difficult at low rates. If precise control over wide ranges is necessary, placing a second, smaller valve in parallel often makes sense.
Nearly all equipment will have a lower operating limit. For both designing and troubleshooting, understand these limits, why they are there, how they behave, and what tradeoffs they may require.
ANDREW SLOLEY is a Chemical Processing contributing editor. You can email him at ASloley@putman.net.