Abstract

Introduction

High pressure spikes typically occur within propellant feedlines during a priming event1. Such an event occurs when
a pyro valve, or latch valve opens, allowing liquid propellant to fill the downstream lines, which are at vacuum
conditions. These spikes are the result of high velocity propellant flowing into a dead-ended line (closed valve). The
impact of the column of liquid propellant hitting the closed valve creates a compression wave, also referred to as
“Water Hammer”. This priming event is a well known phenomenon that has been much studied and analyzed.
Pressure spikes may lead to detonation of the propellant. As a rule of thumb, pressure spikes should not exceed the
proof pressure rating of both the pressurization lines and the flow control components. A possible solution is to
install a fixed flow restriction in the line, which will decrease the liquid impact velocity, thus reducing the water
hammer. The drawback to this approach is that the system will suffer from a pressure loss during normal flow usage.
This paper presents a trade study of four devices: an orifice, a venturi, a flow fuse, and a flow fuse with a dashpot
(slow closing). A flow fuse will sense an abnormally high flow, and close a poppet so that the orifice size is
reduced. This would limit the flow velocity, thus reducing the pressure surge. The poppet would then return to its
full open position during nominal flow, thus not producing any excess pressure drop during normal operation.

A literature search of surge mitigation devices has shown popular use of orifices2,3 and venturis4. A literature search
of the use of a flow fuse in a propulsion system yielded no previous results. The lack of use of such a device may be
due to certain concerns about improper operation during use. This paper will show that if a flow fuse is properly
sized, it can be used during a priming event with great success, and no unintended consequences.

The trade study will size each device to have the same flow area (CdA). This will allow the four devices to produce
the same pressure surge during priming, so that they can be evaluated on their pressure drop during thruster firing.
The pressure and velocity during a priming event, with the proposed flow limiting devices, will be calculated using
well established numerical methods. The pressure and velocity will also be calculated for a thruster firing using the
same methods.

A typical spacecraft propulsion system is shown in Figure 1. Once in orbit an isolation valve or a pyrotechnic valve
opens, creating this priming event, which can be destructive due to the large amount of energy that is released. A
Valcor Thruster Valve shown, in Figure 2, and is used to control the flow of hydrazine to the thruster. The ESEO for
this valve is .075 inch. The valve will provide a flow of 2 gpm to the thruster at the beginning of a mission when the
pressure in the propellant tank is at its highest, which will be 360 psi for this analysis. Since the thruster valve is a
fast acting solenoid valve, a pressure surge will be created at the end of each thruster firing when the valve closes.
The surge will be the largest at the beginning of the mission. Since the system is designed for this surge pressure
from the thruster valve closing, the surge protection devices will be sized to have the same surge pressure during the
priming event. This will be the optimum size (CdA) for each device, in order to limit pressure loss.

Surge Mitigation Devices

The four surge mitigation devices are shown in Figure 3. The orifice and venturi are simple one piece parts.
These parts can be machined into a fitting that can be installed into the tubing, upstream from the isolation valve.
The orifice is sharp edged and has a diameter of .075”. The venture has a throat diameter of .058 inch. A typical loss
factor of 15% is used to calculate the pressure drop across the venturi.

Two flow fuses are shown in figure 3. Item 3) is a flow fuse without a dashpot and will be referred to as “Fast
Closing”. This flow fuse is shown in the open and closed position in Figure 4. Item 4) is the same flow fuse except
with a dashpot and will be referred to as “Slow Closing”. This flow fuse is shown in the open and closed position in
Figure 5. A device can be called “Slow Closing” if it is not fully closed by the time the first reflected wave returns.
The closing time for the slow flow fuse is .060 seconds. Since the flow fuse is located only 5 feet from the tank, the
wave returns in .002 seconds.

The flow fuse is a check valve with its poppet spring loaded open. It is a flow sensitive device in the normal flow
direction. It is a free flow element in the reverse direction. This flow fuse contains only five parts: inlet fitting, outlet
fitting, poppet, sleeve, and spring. The mechanical operation is simple as the poppet is the only moving part and it
does not have any dynamic seals. Under normal conditions, the poppet is spring loaded to the open position. During
this condition and up to the trip point, the force differential created by the flow across the poppet is equal to or less
than the spring force. If the flow increases beyond the trip point, as would occur if a downstream pipe ruptured, then
a large pressure differential across the poppet will counteract the spring force, thus slamming the valve shut. The
same closure would occur in the case of a priming event, when a downstream valve suddenly opens up into an
evacuated line, thus having the same effect as a line rupture. A properly sized flow fuse will be designed with a flow
trip point well above the nominal expected flow. This will guarantee that flow fuse will not accidentally trip, which
would occur if the flow fuse was undersized. A properly sized flow fuse will also be designed with a flow trip point
well below the expected surge flow that would occur in a line breakage or a priming event. This will guarantee that
the flow fuse will close when needed, which would not occur if the flow fuse was oversized.

Each flow fuse that will be used in this study is a 3/8” line size, and has a trip point of 5.4 gpm of water. The
flow fuse will have a poppet that has an open orifice of .200 inch, and a closed (tripped) orifice of .075 inch. To
accomplish reducing the flow during the priming event, the flow fuse used will have a .075 inch ESEO. When the
flow fuse is tripped the flow will be reduced by only allowing flow through the .075 inch ESEO, and no longer
across the .200 inch seat. The flow fuse without the dashpot will be assumed to close instantly when the flow
reaches the trip point. The flow fuse with the dashpot will close in .060 seconds when the flow reaches the trip point.

Modeling Approach

The model parameters are listed in Table 1. A schematic of the system that was modelled is shown in Figure 6.
The system is modeled as a straight tube since the focus of the study is to compare surge mitigation devices and not
the effects of the system complexity on surge pressures. The surge device is located upstream of the isolation valve
and is wetted. This is to avoid any additional surge that would be caused by propellant impacting an unwetted
restriction. The feedline was modeled as a Partial Differential Equation (PDE) by using the Method of
Characteristics (MOC)5. This model divided the feedline into 100 segments. This model provides accurate results by
including the elastic effects of the fluid and the pipe, along with the transient pressure waves created when the flow
fuse, isolation valve, and thruster valve, open and close. The MOC model used the method of Lin and Baker6, who
used the Method of Characteristics (MOC) to treat one-dimensional liquid transients in liquid-full segments, and the
lumped-inertia technique to model the dynamics of partially filled segments. The equations for MOC are shown in
Figure 7. The equations for the partially-filled segment are shown in Figure 8. The simulation assumes that there is
vacuum between the isolation valve and the thruster valve prior to the priming event. Although some systems have a
blanket of low pressure helium downstream to decrease the surge, using a vacuum is conservative in the event that
the helium leaks through the thruster valve prior to priming. The isolation valve and the thruster valve are assumed
to open and close instantly, which is a conservative assumption.

The following is a list of general modeling assumptions:

• 1D axisymmetric flow
• Wave speed is constant (density is constant)
• Mach Number <<1
• Constant pipe diameter
• Pipe is straight, no bends or tees
• Friction modeled using Darcy-Weisbach Formula
• Vaporization of propellant produces a negligible amount of gaseous propellant
• Thruster valve opening and closing time is instantaneous
• Isolation valve opening time is instantaneous
• Flow Fuse closing time is instantaneous without dashpot

Priming Event Simulation Results

Five priming event simulations were run with the MOC Model. The MOC model was run using water as the
liquid propellant, to allow for ease of comparison for future tests. The results are listed in Table 2. Case 1, without a
device, has an extremely high pressure of 3,730 psi, and an extremely high flow of 14.1 gpm, as is shown in Figure
9 and Figure 10. This pressure is high enough to damage the system and detonate the propellant. Case 1 has the
shortest impact time of .180 seconds. It can be seen from Figure 10, that the flow reaches a maximum and then
decreases before impact. Cases 2, 3, and 4, are similar since the orifice, venturi, and closed flow fuse all have the
same CdA. The max pressure for Case 4, 1098 psi, is higher than the max pressures for Case 2 and Case 3. The max
pressure for Case 4 does not occur at the thruster. The max pressure occurs at the flow fuse when the flow fuse
closes, at time = .01 seconds. It should be noted that this max pressure spike was upstream of the flow fuse and
never traveled downstream to the thruster. This is shown in Figure 15 and Figure 16. This closure reduces the max
flow from 5.4 gpm to 2.0 gpm, which produces the max pressure surge for this case. The pressure at the thruster at
impact is 901 psi which is shown in Figure 17. The max pressure for Case 5 is 901 psi, and occurs when the flow
impacts the thruster. This pressure is the same as Case 2. It can be seen from Figure 19 that when the flow fuse
closes slowly the pressure at the flow fuse never exceeds 500 psi. It can be seen from Figure 20 that the max flow
reached 9.5 gpm while the flow fuse was closing, and reached 2 gpm by the time the flow fuse had closed, without
any additional pressure surge.

Thruster Firing Simulation Results

Five thruster firing simulations were run with the MOC Model. The results are shown in Table 3. The MOC
model was run using water as the liquid propellant, to allow for ease of comparison to future tests. The simulations
were run to determine the pressure transients that occur during the thruster firing in a fully primed system. The
thruster valve ESEO is .075 inch. The thruster valve will cause a pressure surge when the thruster valve is closed.
The thruster valve was opened at time = 0 second, and closed at time = .2 seconds for all five cases. During this time
a steady state flow was established that was used to determine the pressure drop in the feedline for all five cases. It
should be noted that maximum pressure for the thruster valve closing for all devices is very similar to the maximum
pressure for the priming event for all devices. Case 6, without any device, has the highest pressure of 931 psi. Cases
7 has the lowest pressure. This is expected because the flow is 1.5 gpm, which is less than all the other cases. For
Case 9 and Case 10, it should be noted that the flow fuse stayed open as expected, and the flow never exceeded 2.0
gpm, which is well below the flow fuse trip point of 5.4 gpm

Conclusions

The results of all the simulation cases are listed in Table 4. In general the pressure surges for each device were
similar, and this was due to the sizing criteria selected for the trade study. The selection of the best device should
then be based on pressure loss during thruster firing. Figure 33 shows the pressure loss for all devices at the
beginning of the mission (BOL). The pressure drop occurs 5 feet from the tank, which is where all the devices were
located. The pressure loss for both flow fuses is the lowest at 6.6 psi. The venturi is a better option than an orifice,
but still not as good as the flow fuses. The closed ESEO for the flow fuse without the dashpot could not be reduced
any further since this will increase the pressure surge at the flow fuse due to its fast closing. The advantage of the
slow closing flow fuse is that the priming event surge pressure could still be greatly reduced without any change to
the 6.6 psi pressure drop during thruster firing by decreasing the flow fuse closed ESEO. This is why the optimum
surge mitigation device is the flow fuse with the dashpot. The addition of the dashpot to the flow fuse will also make
it more reliable, since the dashpot will eliminate unintended closures caused by short duration pressure spikes.

It is unknown why flow fuses have not been previously used in propellant systems. Some engineers may have
misgivings about using these devices due to stories about unintended flow fuse closings, which cause problems.
Almost all flow fuse problems can be traced to improperly sized devices. These simulation models provide an ideal
tool for evaluating surge pressure effects in propellant feedlines, and have shown that a properly sized and analyzed
flow fuse will reliably perform its intended task with confidence.

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