Installation Effects

Abstract

Most flowmeters are affected by how and where they are installed. Generally meters are calibrated (or designed in accordance with ISO 5167) to give a predictable performance when installed where the flow profile approximates to a fully developed flow profile at the Reynolds number of the flow. A flow profile that is disturbed by an upstream fitting may cause a change in the performance. All differential-pressure meters change their performance if flow disturbance is introduced, with orifice plates being particularly affected. Orifice plates do, however, have the advantage over many other meters in that the effect of installation is often known. There are huge amounts of data on installation effects on orifice plates, but very limited data on installation effects on Venturi tubes. The effect of upstream fittings and pipework is considered in terms of peakiness of profile, asymmetry and swirl. The basis on which the straight lengths in ISO 5167:2003 were derived is described. In practice it is necessary to follow ISO 5167-2:2003/ISO 5167-4:2003 to provide minimum straight lengths. Where it is not possible to conform to Table 3 of ISO 5167-2:2003/Table 1 of ISO 5167-4:2003, alternative options, some including the use of flow conditioners, are described. The basis and use of the flow conditioner test in ISO 5167-1:2003 are described. Limits on pulsation are described.

Appendix 8.A: Swirl Decay

Swirl decay was calculated by Reader-Harris (1994). Computational work showed that beyond a short distance downstream of a swirl-inducing installation the swirl is proportional to exp(−ψx/D), where x is the distance down the duct, and that the decay rate ψ is equal to 1.07λ, where λ is the friction factor in the pipe (see Sect. 1.5). In Fig. 8.A.1 the decay rate in this computational model is compared with experimental data (Baker and Sayre 1974; Kitoh 1965; Kreith and Sonju 1965; Mattingly and Yeh 1990; McManus et al. 1985; Mottram and Rawat 1986; Murakami et al. 1976; Senoo and Nagata 1972).

Fig. 8.A.1

The decay rate of swirl in a pipe

On the basis of the computational model, if the pipe Reynolds number is 6 × 10⁶ and R _a /D is 10⁻⁵, after 200 diameters an 18° swirl (a typical value immediately downstream of two bends in perpendicular planes) will decay to 2°.