An analytical sampling system relies on transport lines to convey fluid from a process stream to an analyzer. The goal is to supply the fluid as efficiently as possible to avoid lengthy time delays between sampling and analysis as well as to minimize sample waste.
Achieving optimum efficiency is well within your reach. However, success depends on choices you make when designing the sampling system. One of the earliest is about the type of sample transport lines to use to divert flow from the process stream to the analyzer. You can select between two basic types: single-line systems and fast-loop systems. Making an optimal choice requires some knowledge of the design challenges in each case. Table 1 lists some general guidelines for when to choose a single-line system or a fast-loop one. You need not follow all of these recommendations and also may decide to consider other factors. The key is to optimize your selection to fit your real process application.
Let’s walk through some selection criteria — including sample disposal needs, pressure drop and time delay — that will help you choose the right sample transport line for your application.
You can use a single-line transport system (Figure 1) only if you don’t intend to return the sample to the process and can dispose of it properly. As shown in Figure 2, the sample is extracted from the process line, transported to a conditioning system if necessary and directed to the analyzer before being disposed of via a vent or drain. As we’ll discuss, multiple variables may force you to use a fast-loop system instead.
Disposal likely won’t pose a problem if the sample is a gas and the full line flow can go to a flare, furnace or stack. For example, you can vent an innocuous sample such as a nontoxic stack gas to the atmosphere. However, many process plants are reducing flaring due to pollution and material waste concerns. These constraints may prevent the flaring of certain gases and, so, may promote returning them to the process via a fast loop.
Single-line transport is less common for liquid samples, mostly due to disposal problems. You readily can send a low-purity water sample down the drain. However, a less friendly liquid may require an oily sewer, a ground flare, an incinerator or a sample recovery tank. If you can’t find a suitable place to dispose of the sample, you must return the liquid to the process via a fast-loop system.
When using a single-line system, you may experience an unacceptable analysis time delay (i.e., one exceeding the 60 seconds or less generally considered satisfactory) that will necessitate moving to a fast-loop system. Tables 2 and 3 offer some typical data for achieving a 60-sec transport time using air or water at varying line lengths and sizes; data for other gases and liquids generally will be similar.
The pressure drop in gas lines usually is negligible, so single lines often work well up to about 100 m. Above this length, the flow rate required to achieve a 60-sec analyzer response may become wasteful because a lot of sample material will be vented. You may have to consider a sample recovery system or a fast loop back to process.
For liquids, pressure drop increases rapidly with line length and can become excessive in single lines longer than about 60 m. If pressure drop appears close to the maximum acceptable, complete a full pressure drop calculation before deciding on a single line. You may need a fast loop instead.
A single line is preferred if your sampling system relies on a vaporizer at the tap. A fast loop generally doesn’t suit such situations.