Study on Calculation of the hottest dry powder fir

Research on Calculation of dry powder fire extinguishing system

in order to provide basis for the design of dry powder fire extinguishing system and the compilation of design specifications, this paper studies the calculation of relevant parameters of dry powder fire extinguishing system and dry powder transportation management. The results are as follows:

1 Determination of pipe diameter

the inner diameter of dry powder fire extinguishing system pipe is determined by the volume of gas-solid two-phase fluid passing through and the appropriate flow velocity, The former can be calculated according to the amount of dry powder to be transported in the pipeline, the type of driving gas, the driving gas coefficient, the ambient temperature and the pressure in the pipeline; The latter needs to be determined by experiment

in order to ensure that the dry powder in the pipeline of the dry powder fire extinguishing system is not separated from the driving gas, the two-phase flow of dry powder and driving gas must maintain a certain flow rate, which requires that the transmission rate of dry powder in the pipeline should not be less than the minimum allowable value qmin. Based on this principle, in order to establish the specific relationship between the inner diameter of the pipeline and the conveying rate of dry powder, the British Standard Recommended test data [1] is quoted. The British standard points out that in order to ensure that dry powder does not deposit in the pipeline, the minimum conveying rate qmin of dry powder in the pipeline with an inner diameter of 27mm is required to be 1.5/s. From this, the relationship between the inner diameter of the pipe D and the conveying rate Q of dry powder in the pipe is obtained:

D =kd · (q) 1/2=22 (q), which makes us clearly realize that 1/2 (1)

in the formula: KD pipe diameter coefficient

for comparison, the data of the United States and Japan are listed in Table 1 []

the data in Table 1 shows that both the data of the United States and Japan are very close to the data of the United Kingdom, which further confirms the reliability of formula (1)

it should be pointed out here that the maximum pipe diameter is calculated by using formula (1). According to the needs, the actual pipe diameter value should be taken as an appropriate value smaller than the calculated value. According to the requirements of economic flow rate in the pipeline, the finally determined pipe diameter should not be less than half of the calculated value

2 determination of the working pressure of the system

it is not expected that the dry powder fire extinguishing system will reduce the price. The working pressure of the pipeline is a necessary condition to ensure the normal operation of the dry powder fire extinguishing system, which usually includes the pressure lost in the pipeline, the working pressure of the nozzle, the relationship between qmin and D in Table 1 of the United States and Japan due to different positions and heights, the average pipe diameter coefficient Kd value pressure difference, etc. In general, the latter two items are easy to determine, and there is no need to discuss more. Here we mainly analyze the pressure loss in the pipeline

what flows in the pipeline of the dry powder fire extinguishing system is a gas-solid two-phase fluid, which is the same as the gas transmission of powdery materials in terms of the transmission object, so the pressure loss in the pipeline must be similar

△ P = △ PQ + △ PZ (2)

in the formula: △ p - pressure loss in the pipeline, PA

△ PQ pressure loss caused by gas flow, PA, i.e.

△ PQ= λ q·l· ρ q· υ Q2/(2D) (3)

△ PZ pressure loss caused by powdery material carried by gas, PA, i.e.

△ PZ= λ q·l· ρ q· υ q2/(2 μ· d) (4)

so there are:

△ p=（ λ q+ λ z/μ) l· ρ q· υ Q2/(2D) (5)

or: △ p/l=（ λ q+ λ z/μ)·ρ q· υ Q2/(2D) (6)

where: △ p/l - pressure loss per unit length of pipe, all these pa/m

λ Q-drag loss coefficient of driving gas:

λ Z-additional resistance loss coefficient of dry powder

μ- The Chinese market of driving gas coefficient has great growth potential

ρ Q-density of driving gas, kg/m3

υ Q-the flow velocity of the driving gas in the pipeline, m/s

d- pipe inner diameter, m

l- pipe length, M

due to the large flow velocity of the driving gas in the pipeline, the resistance loss coefficient along the way λ Q is calculated according to the condition of hydraulic rough pipe, namely:

λ Q=[(1.lg (0.39/d)] -2 (7)

where 0.39 is the absolute roughness of galvanized steel pipe [6], mm

for different pipe diameters, the results calculated by formula (7) are shown in Table 2