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Valve Design Exercise

The flow rate of cooling fluid through a system is regulated by a valve. The pressure generated by the pump is constant.

$$\Delta P_{pump}=100 \mathrm{bar}$$

The cooling fluid has a specific gravity of 1.1. As a first step, characterize the valve without the system pressure drop. In a second step, determine the best valve trim for a linear relationship between lift and flow for the installed valve.

Part I: Valve Characterization

Plot the relationship between flow `q` and valve lift `l` for the valve characterized by the design equation.

$$q = C_v f(l) \sqrt{\frac{\Delta P_v}{g_s}}$$

with `C_v = 2` and either a linear lift function `f(l)=l` or an equal percentage lift function `f(l)=R^{l-1}` with `R=20`.

Solution, Part I

import numpy as np
import matplotlib.pyplot as plt

## Lift functions for two different valve trim types
def f_lin(x):
    return x             # linear valve trim
def f_ep(x):
    R = 20
    return R**(x-1)      # equal percentage valve trim (R = 20-50)

lift = np.linspace(0,1)  # equally spaced points between 0 and 1

plt.figure(1)            # create new figure
plt.title('Valve Performance - Not Installed')
plt.subplot(2,1,1)                # 2,1 subplot with 1st window
plt.plot(lift,f_lin(lift),'b-')   # linear valve
plt.plot(lift,f_ep(lift),'r--')   # equal percentage
plt.ylabel('f(l)')
plt.legend(['Linear Valve Trim','Equal Percentage Valve Trim'])

g_s = 1.1                # specific gravity of fluid
def q(x,f,Cv,DPv):
    return Cv * f(x) * np.sqrt(DPv/g_s)   # flow through a valve

## Intrinsic valve performance
# no process equipment - all pressure drop is across valve
DPt = 100 # Total pressure generated by pump (constant)
Cv = 2    # Valve Cv
flow_lin = q(lift,f_lin,Cv,DPt)  # flow with linear valve
flow_ep  = q(lift,f_ep,Cv,DPt)   # flow with equal percentage

plt.subplot(2,1,2)               # 2,1 subplot with 2nd window
plt.plot(lift,flow_lin,'b-')     # plot linear valve response
plt.plot(lift,flow_ep,'r--')     # plot equal percentage valve
plt.ylabel('Flow')
plt.legend(['Linear Valve Trim','Equal Percentage Valve Trim'])
plt.xlabel('Fractional Valve Lift')
plt.show()

Part II: Installed Valve

The pressure drop across the system (without the valve) follows the following correlation between pressure drop `\Delta P_{system}` and flow rate `q`.

$$\Delta P_{system} = 2 q^2$$

Plot the relationship between flow `q` and valve lift `l` for the system and valve with `C_v = 2` and either a linear lift function `f(l)=l` or an equal percentage lift function `f(l)=R^{l-1}` with `R=20`.

Solution, Part II

The first step in this solution is to relate the pressure drop across the valve and the system to the pressure generated by the pump.

$$\Delta P_{pump} = \Delta P_v + \Delta P_{system}$$

with

$$\Delta P_{pump} = 100$$

$$\Delta P_v = g_s \left(\frac{q}{C_v f(l)}\right)^2$$

$$\Delta P_{system} = 2 q^2$$

This gives a combined equation in terms of lift `l` and flow `q`.

$$100 = g_s \left(\frac{q}{C_v f(l)}\right)^2 + 2 q^2$$

Algebraic manipulation is then used to obtain flow as a function of lift `q(l)` for the valve and system.

$$q(l) = \sqrt{\frac{100\left(C_v f(l)\right)^2}{(g_s + 2 \left(C_v f(l)\right)^2)}}$$

import numpy as np
import matplotlib.pyplot as plt

DPt = 100    # Total pressure generated by pump (constant)
Cv = 2       # Valve Cv
g_s = 1.1    # specific gravity of fluid
lift = np.linspace(0,1)  # equally spaced points between 0 and 1

## Lift functions for two different valve trim types
def f_lin(x):
    return x             # linear valve trim
def f_ep(x):
    R = 20
    return R**(x-1)      # equal percentage valve trim (R = 20-50)

## Installed valve performance
# pressure drop across the system (without valve)
c1 = 2
def DPe(q):
    return c1 * q**2

# valve and process equipment flow with 100 bar pressure drop
def qi(x,f,Cv):
    return np.sqrt((Cv*f(x))**2*DPt / (g_s + (Cv*f(x))**2 * c1))

# Process equipment + Valve performance
flow_lin = qi(lift,f_lin,Cv)  # flow through linear valve
flow_ep  = qi(lift,f_ep,Cv)   # flow through equal percentage valve

plt.figure(2)
plt.title('Valve Performance - Installed')
plt.subplot(3,1,1)
plt.plot(lift,flow_lin,'b-',label='Linear Valve')
plt.plot(lift,flow_ep,'r--',label='Equal Percentage Valve')
plt.plot([0,1],[0,6.5],'k-',linewidth=2,label='Desired Profile')
plt.legend(loc='best')
plt.ylabel('Flow')

plt.subplot(3,1,2)
plt.plot(lift,DPt-DPe(flow_lin),'k:',linewidth=3)
plt.plot(lift,DPe(flow_lin),'r--',linewidth=3)
plt.legend(['Linear Valve','Process Equipment'],loc='best')
plt.ylabel(r'$\Delta P$')

plt.subplot(3,1,3)
plt.plot(lift,DPt-DPe(flow_ep),'k:',linewidth=3)
plt.plot(lift,DPe(flow_ep),'r--',linewidth=3)
plt.legend(['Equal Percentage Valve','Process Equipment'],loc='best')
plt.ylabel(r'$\Delta P$')
plt.xlabel('Lift')

plt.show()