91.414 Raw gaseous exhaust sampling and analytical system description.§ 91.414 Raw gaseous exhaust sampling and analytical system description.
(a) Schematic drawing. An example of a sampling and analytical system which may be used for testing under this subpart is shown in Figure 4 in appendix B of this subpart. All components or parts of components that are wetted by the sample or corrosive calibration gases shall be either chemically cleaned stainless steel or inert material (e.g., polytetrafluoroethylene resin). The use of “gauge savers” or “protectors” with nonreactive diaphragms to reduce dead volumes is permitted.
(b) Sample probe. (1) The sample probe shall be a straight, closed end, stainless steel, multi-hole probe. The inside diameter shall not be greater than the inside diameter of the sample line + 0.03 cm. The wall thickness of the probe shall not be greater than 0.10 cm. The fitting that attaches the probe to the exhaust pipe shall be as small as practical in order to minimize heat loss from the probe.
(2) The probe shall have a minimum of three holes. The spacing of the radial planes for each hole in the probe must be such that they cover approximately equal cross-sectional areas of the exhaust duct. The angular spacing of the holes must be approximately equal. The angular spacing of any two holes in one plane may not be 180° ±20° (i.e., section C-C of Figure 1 in appendix B of this subpart). The holes should be sized such that each has approximately the same flow. If only three holes are used, they may not all be in the same radial plane.
(3) The exhaust gas probe must be located in a position which yields a well mixed, homogeneous sample of the engine exhaust. The probe must extend radially through the exhaust duct prior to where the exhaust mixes with the cooling water. The cooling water flow may be rerouted if necessary to obtain an emission sample provided that the modification has no significant effect on the performance or emissions characteristics of the engine. The probe must pass through the approximate center and must extend across at least 80 percent of the diameter of the duct. The exact position of the probe may vary from engine family to engine family.
(c) Sample transfer line. (1) The maximum inside diameter of the sample line shall not exceed 1.32 cm.
(2) If valve V2 in Figure 1 of appendix B of Subpart D of this part is used, the sample probe must connect directly to valve V2 in Figure 1 of appendix B of subpart D of this part. The location of optional valve V2 may not be greater than 1.22 m from the exhaust duct.
(3) The location of optional valve V16 in Figure 1 of appendix B of subpart D of this part may not be greater than 61 cm from the sample pump. The leakage rate for this section on the pressure side of the sample pump may not exceed the leakage rate specification for the vacuum side of the pump.
(d) Venting. All vents including analyzer vents, bypass flow, and pressure relief vents of regulators should be vented in such a manner to avoid endangering personnel in the immediate area.
(e) Any variation from the specifications in this subpart including performance specifications and emission detection methods may be used only with prior approval by the Administrator.
(f) Additional components, such as instruments, valves, solenoids, pumps, switches, and so forth, may be employed to provide additional information and coordinate the functions of the component systems.
(g) The following requirements must be incorporated in each system used for raw testing under this subpart.
(1) Take the sample for all components with one sample probe and split it internally to the different analyzers.
(2) Heat the sample transport system from the engine exhaust pipe to the HC analyzer for the raw gas sampling method as indicated in Figure 1 in appendix B of subpart D of this part. The NOX analyzer for the raw gas sampling method may be heated as indicated in Figure 1 in appendix B of subpart D of this part. The HC analyzer and the NOX analyzer for the dilute sampling method may be heated as indicated in Figure 1 in appendix B of subpart D of this part.