1. Introduction

Davit is the device used to lift and move heavy things around. It can be stationary or portable, manual or powered by motors, and usually have the configuration shown in the Figure 1 (left). As you can see, it’s a quite simple device.

Figure 1 - Two types of davit: one used to lift equipment (left) and other used to lift pressure vessel cover (right)

Figure 1 – Two types of davit: one used to lift equipment (left) and other used to lift pressure vessel cover (right)

The design of davit for pressure vessels are described in the Procedure 9-1 from Pressure Vessel Design Manual[1], which according to references section in the book was based in paper published by Magnusson from Fluor Engineers entitled “Design of Davits” (I couldn’t find such paper in the internet, so I think it may be a private document from that company) and in the well-known “Roark’s Formulas for Stress and Strain”[2] book.

2. Types of Davit

The book specifies 3 types of rigging as shown in the Figure 2:

Davit Design for Pressure Vessel - Kezar Engineering

Figure 2 – Rigging Types

The engineer must choose the best type that suits his needs. If there’s enough space to install a chain or ratchet hoist, then choose type 2 (this type will also be used when designing a whell handle with a threaded rod to lift the head of the pressure vessel), if the the cranck or motor is installed somewhere else, choose type 1…

3. Calculation

Variables:

C_v = vertical impactor factor
C_h = horizontal impact factor
f_a = axial stress, MPa
f_b = bending stess, MPa
f_h = horizontal force, N
f_v = vertical force, N
F_a = allowable axial stress, MPa
F_b = allowable bending stress, MPa
F_r = radial load, N
F_re = equivalent radial load, N
F_y = minimum specified yield stress, MPa
M_1 = bending moment in mast at top guide or support, Nm
M_2 = maximum bending moment in curved davit, Nm
M_3 = bending moment in boom, Nm
M_x = longitudinal moment, Nm
M_\phi = circumferential moment, Nm
W_1 = weight of boom and brace, N
W_D = total weight of davit, N
\alpha, \beta, K = stress coefficients
P = axial load, N
I = moment of inertia, mm^4
A = cross-sectional area, mm^2
Z = section modulus, mm^3
r = least radius of gyration, mm
t_p = wall thickness of pipe davit, mm
a = outside radius of pipe, mm

References   [ + ]

1.Moss, D. and Basic, M. (2013). Pressure Vessel Design Manual. 4th ed. Amsterdam: Butterworth-Heinemann, p. 558
2.Young, W. C., Budynas, R. G., Sadegh A. M. (2011). Roark’s Formulas for Stress and Strain. 8th ed. McGraw-Hill Education